FMU-crashes, proper debugging?

this might be a somewhat stupid question.

For a certain time, I have the problem that the pixhawk 1 crashes from time to time while flying in mission mode (Usually in loiter to alt). After that, I still have manual control. The FMU restarts but is obviously useless after an airborn restart. The log stops while I am in air. I could not find any indications that there was something wrong in the log file. Sadly, I did not get any hardfault log.

I have implemented a few new things into an older stable branch.
Meaning, it is a custom firmware and I cannot expect you helping me out with it.

However, there is something that would help me a lot. Is there a possibility that the fmu crashes without causing a hardfault log?
What imaginable way can restart the FMU but still leave you manual control?
Could there be any watchdogs on px4 level or even on STM32 level which could trigger a reset?
Are there any recommended approaches to debug such a behavior in a better way?
Thanks a lot!
Best Gian

Are there any ideas? Now I am using a optiguard ultra as backup battery. Thus, I think it has to be something software based. But why aren’t there any crash logs?