I still use FP Open and had similar problems on my last multi-day bike tour. I used “Locus Map” for navigating and was charging my FP2 with a powerbank. After several reboots day after day I recognised a pattern: every time the charging status was at ~92% the reboots started happening - never with lower power status. This was 100% reproducable: I tried to keep an eye on the battery percentage and stopped charging when getting close to 90%. That prevented reboots on multiple days until I again didn’t pay enough attention and accidentally charged above the critical point: instant reboot.
Also, I don’t think it’s the GPS or the routing per se but more some sort of “heavy” usage, as reboots also happen sometimes to me when I have the phone plugged in the charger at home and use it for other tasks (no GPS activated!). To my recollection it also happens in those cases when the phone is merely fully charged, but cannot confirm this for sure, as I mostly charge over night.
In other circumstances then the described “charging reboots” I very rarely experience any other reboots.