Update:
I noticed another possible clue. The NAS clock is sometimes drifting behind the correct time even though external NTP servers are reachable.
Checking NTP status shows:
sync_local
refid=LOCAL(0)
instead of using an external NTP source.
The external NTP servers are reachable, but the system is selecting LOCAL(0), and the NAS clock was around 20-30 seconds behind the actual time.
This may explain the previous false UPS notification, as the logs showed:
Clock change detected
upsmon: Processing OS wake-up after sleep
ups_notify_on_power_restored
The UPS itself has not lost power. It looks more like a TOS7 time synchronisation issue.