@d4f0 @messayisto
thanks for your replies, I'm beginning to think that there is nothing wrong neither in force-adb nor in systemd (!) and the problems comes from the device tree of the FP5, that is, the kernel configuration.
Changing force-adb could be an easy work around but mostly wrong.
I'll try to explore the FP5 kernel code to see if I can find the problem all by myself (enthusiasm level: )
Edit: created this gitlab issue to make the problem more visible.