How much survives an install/reboot cycle?

How much of a running system's state survives a reboot? I used to think
the answer was "nothing", but from time to time a second reboot behaves
a  little differently from the previous one. 

The most recent example was an update to bpf.c: Prior to the update an
armv7 system had been inclined to drop ssh connections left up for days.
After updating and running a build/install cycle the behavior persisted,
but since a second reboot with no intentional changes it has stopped.

I've not tampered with nextboot, so I don't think that's it. Maybe I'm
just imagining imagining things.... 

Thanks for reading,

bob prohaska