I remember issues with at least the mood background for very early versions of the operating system of the Fairphone 1. The Fairphone 2 being affected would be news to me.
Anyway: apparently an update is on the way, but Fairphone is looking for tech-savvy volunteers to test the update first. I’m mentioning it here as it specifically looks for people experiencing random reboots. If the description below isn’t warning enough, do realise that things do go wrong in test versions - though if you can explain fastboot and ADB to an 8-year old you will know this anyway.
(Click through to the topic for details on how to apply if interested)