After this month's Patch Tuesday, I finally bit the bullet and updated from 7.4.42 to 7.6.22, BOINC still being installed as a service. The update itself didn't ask for a reboot, so kept going with it like that for days and next time I rebooted it was still fine. However, after the next 2 reboots after that (actually one was a shutdown), when I booted back up and checked event logs, I saw it reported that the previous shutdown had been unexpected, which it obviously wasn't. Then I didn't reboot again until today, when I rebooted twice and shut down BOINC (and running projects) before doing so, the result being that there was no other unexpected shutdown log entry.

So I'm guessing something about how BOINC 7.6 handles running as a service causes that to happen. More specifically, think it has to do with how it logs on, because there was that first normal reboot after updating, which had 7.6.22 when I logged off but it was 7.4.42 that started when I logged on. Nothing else I can identify that may be causing it.