After I wrote the message above I realised this is another topic in fact, I hope you don't mind.
To explain a bit more what happens, here is some debug output. Each 10 seconds an increasing counter is sent to the gateway, and after each sent message the TX power is printed on the screen.
46219 TSM:UPL:OK
46252 TSM:READY:ID=31,PAR=0,DIS=1
54607 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:4
Power is 3
64618 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:5
Power is 2
74629 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:6
Power is 1
85655 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:7
Power is 1
94650 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=OK:8
Power is 0
105168 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:9
Power is -1
114671 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:10
Power is -2
124682 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:11
Power is -3
135708 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:12
Power is -3
144703 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=OK:13
Power is -4
154714 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:14
Power is -5
164724 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:15
Power is -6
174735 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:16
Power is -7
184745 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:17
Power is -8
195772 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:18
Power is -8
204980 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=OK:19
Power is -9
214990 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:20
Power is -10
225804 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:21
Power is -10
235438 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=OK:22
Power is -11
245825 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:23
Power is -11
255836 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=NACK:24
Power is -11
265846 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=2,st=NACK:25
Power is -11
275857 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=3,st=NACK:26
Power is -11
285868 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=4,st=NACK:27
Power is -11
Here are a couple of NACKs, so this is on the limit of TX power IMO. Still one succesfull one, which decreases the power even more.
294862 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=5,st=OK:28
Power is -12
305889 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:29
Power is -12
315899 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=NACK:30
Power is -12
325910 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=2,st=NACK:31
Power is -12
335921 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=3,st=NACK:32
Power is -12
345931 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=4,st=NACK:33
Power is -12
355942 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=5,st=NACK:34
Power is -12
Five failed messages and the node starts finding its parent. But also that fails.
356024 !TSM:READY:UPL FAIL,SNP
356073 TSM:FPAR
356089 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=6,st=OK:
358203 !TSM:FPAR:NO REPLY
358219 TSM:FPAR
358252 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
360366 !TSM:FPAR:NO REPLY
360382 TSM:FPAR
360415 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
362512 !TSM:FPAR:NO REPLY
362528 TSM:FPAR
362561 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
364675 !TSM:FPAR:FAIL
364691 TSM:FAIL:CNT=1
364724 TSM:FAIL:DIS
364740 TSF:TDI:TSL
364888 !TSF:SND:TNR
Power is -12
374784 TSM:FAIL:RE-INIT
374800 TSM:INIT
374833 TSM:INIT:TSP OK
374865 TSM:INIT:STATID=31
374898 TSF:SID:OK,ID=31
374915 TSM:FPAR
374947 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
375046 !TSF:SND:TNR
Now the Transportlayer is re-initialised and the power level goes to 5, which makes the messages being ACKed again. This starts the process all over again.
Power is 5
375734 TSF:MSG:READ,0-0-31,s=255,c=3,t=8,pt=1,l=1,sg=0:0
375799 TSF:MSG:FPAR OK,ID=0,D=1
377044 TSM:FPAR:OK
377061 TSM:ID
377077 TSM:ID:OK
377110 TSM:UPL
377192 TSF:MSG:SEND,31-31-0-0,s=255,c=3,t=24,pt=1,l=1,sg=0,ft=0,st=OK:1
377913 TSF:MSG:READ,0-0-31,s=255,c=3,t=25,pt=1,l=1,sg=0:1
377978 TSF:MSG:PONG RECV,HP=1
378028 TSM:UPL:OK
378044 TSM:READY:ID=31,PAR=0,DIS=1
385105 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:37
Power is 3
395116 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:38
Power is 2
405127 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:39
Power is 1
415137 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:40
Power is 0
425148 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:41
Power is -1
435159 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:42
Power is -2
445169 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:43
Power is -3
455180 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:44
Power is -4
465207 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:45
Power is -5
475201 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:46
Power is -6
485720 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:47
Power is -7
495222 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:48
Power is -8
505446 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:49
Power is -9
516259 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:50
Power is -9
525254 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=OK:51
Power is -10
536281 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:52
Power is -10
546291 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=NACK:53
Power is -10
556302 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=2,st=NACK:54
Power is -10
565510 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=3,st=OK:55
Power is -11
576323 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:56
Power is -11
586334 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=NACK:57
Power is -11
596344 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=2,st=NACK:58
Power is -11
606355 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=3,st=NACK:59
Power is -11
615989 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=4,st=OK:60
Power is -12
626376 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=NACK:61
Power is -12
636387 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=1,st=NACK:62
Power is -12
646397 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=2,st=NACK:63
Power is -12
656408 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=3,st=NACK:64
Power is -12
666419 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=4,st=NACK:65
Power is -12
676429 !TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=5,st=NACK:66
Power is -12
676511 !TSM:READY:UPL FAIL,SNP
676560 TSM:FPAR
676577 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=6,st=OK:
678690 !TSM:FPAR:NO REPLY
678707 TSM:FPAR
678739 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
680853 !TSM:FPAR:NO REPLY
680869 TSM:FPAR
680902 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
682999 !TSM:FPAR:NO REPLY
683016 TSM:FPAR
683048 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
685162 !TSM:FPAR:FAIL
685178 TSM:FAIL:CNT=1
685211 TSM:FAIL:DIS
685228 TSF:TDI:TSL
685375 !TSF:SND:TNR
Power is -12
695271 TSM:FAIL:RE-INIT
695287 TSM:INIT
695320 TSM:INIT:TSP OK
695353 TSM:INIT:STATID=31
695386 TSF:SID:OK,ID=31
695402 TSM:FPAR
695435 ?TSF:MSG:SEND,31-31-255-255,s=255,c=3,t=7,pt=0,l=0,sg=0,ft=0,st=OK:
695533 !TSF:SND:TNR
Power is 5
696369 TSF:MSG:READ,0-0-31,s=255,c=3,t=8,pt=1,l=1,sg=0:0
696434 TSF:MSG:FPAR OK,ID=0,D=1
697548 TSM:FPAR:OK
697565 TSM:ID
697581 TSM:ID:OK
697614 TSM:UPL
697696 TSF:MSG:SEND,31-31-0-0,s=255,c=3,t=24,pt=1,l=1,sg=0,ft=0,st=OK:1
698236 TSF:MSG:READ,0-0-31,s=255,c=3,t=25,pt=1,l=1,sg=0:1
698302 TSF:MSG:PONG RECV,HP=1
698335 TSM:UPL:OK
698351 TSM:READY:ID=31,PAR=0,DIS=1
705593 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:69
Power is 3
715603 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:70
Power is 2
725827 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:71
Power is 1
735641 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:72
Power is 0
745635 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:73
Power is -1
755646 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:74
Power is -2
765657 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:75
Power is -3
775667 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:76
Power is -4
785678 TSF:MSG:SEND,31-31-0-0,s=22,c=1,t=23,pt=1,l=1,sg=0,ft=0,st=OK:77