I had this reboot issue every time I came home (since the upgrade). Within the first 15 minutes after I arrived home the phone rebooted. (every day!).
My home wifi supports 2.4 GHz and 5GHz.
Since I don’t want to use 2.4GHz anyway, I switched it off on my FP3 under:
Wi-Fi Preferences, Wi-Fi frequency band, which I set to 5 GHz only.
My phone has not rebooted since (I did this 10 days ago).