It sounds like you used a Back-UPS unit instead of a Smart-UPS, which I’ve heard switches to battery 10x faster than the Back-UPS.
After learning this I won’t use a Back-UPS for anything where meaningful data loss might happen. They’re great for lights, cable modems, WiFi routers, etc., though.
I was doing turn off tests with my NAS attached and it was switching correctly and NAS was not going down, I had couple RPis connected also not going down and notifications from NUT were in logs. Where my NAS was NUT server and RPis had NUT client.
It was that one day when UPS malfunctioned in a bad time. Where my electricity provider was more reliable than UPS I owned. So bad risk management on my side.