@Yveaux Thanks for your reply. I've done a lot of debugging today. And I guess I found the issue.
It's definitely related to the radio power consumption as you suggested.
I'm currently using the new RFM69 driver, with ATC enabled by default. As long as the power level stays below -4 dB, everything works fine. If the gateway is powered off, the radio power level automatically increases to 100% and the PIR behaves weird. It's always HIGH.
By rebooting the gateway, the node relink itself and gradually lowers its power. As before, when it goes below -4dB the PIR starts to work again.
Since it's a battery powered device I decided to set
#define MY_RFM69_MAX_POWER_LEVEL_DBM (-4)
at the beginning of the sketch file. Even if the gateway fails (shutdown) the PIR does not get HIGH anymore. All the measurements work fine now by limiting the max power to the radio.
By doing so all the sleeps of 1000ms after each send are not needed anymore.