LineageOS 15.1 spontaneous reboots
I've been watching it and 2 times it has rebooted I sat in my usual places. 2 bars of wifi and I was on Facebook. WiFi dropped and facebook froze and phone was unresponsive. then black screened and rebooted. Both times it did it at the same spot and the same way. Other times at the spot wifi drops and no reboot. Other times I am driving and the phone is sleeping and reboots. Only way I know is the voicemail notification ding when the phone reboots.
Yes, it's a pretty terrible defect that I remember to be there since LineageOS 14.1.
Another way to know that your phone has rebooted is to set a PIN. If you are asked to reenter the PIN, and before you do that there is a message on the screen, in the PIN entry field, saying, "PIN required after device restarts", this means that the phone has rebooted.
I cannot tell whether the cause is always related to Wi-Fi. An extensive test with Wi-Fi disabled would show that. But since the reboots happen in all kinds of situations, I doubt that Wi-Fi is the only trigger.