@yveaux I think this was the issue.
I just found that
#define MY_MAC_ADDRESS 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xAD
will specify the MAC.
After adding it, no more issues. Changed back to same topics on both gateways, seems this is no problem.
Even the gateways received different IPs via DHCP, they had the same MAC. Haven't seen an issue in any logs.
I am not sure about long term, but short it works now. Let me see if it is a permanent fix.
Still pretty strange that I receive different IPs from my router for the same MAC.
Sometimes these are the easy fixes...
Thanks for your help!