I’m having the same problem. It started occurring sometime last night (UK time) while I was asleep. FP3+ on Honest Mobile. I do not have a second user on my phone.
Unfortunately, the phone doesn’t stay on long enough for me to try disabling Google Play Services.
Will that work when the phone only stays on for about 8 seconds before rebooting? That doesn’t seem like long enough for the phone to load the necessary software, connect to the network, recognise that an update exists, download that update, and install that update, before the phone forcibly restarts again.