Did you ever find out what the issue was with your UPS? I only implemented my 1st UPS setup recently. I've tested that everything shuts down gracefully in a controlled environment, but clearly the UPS is a potential failure point.
Discussion
I'm not sure what happened. Buy fortunately, I cleared step 2 of this guide: https://www.apc.com/us/en/faqs/FA158810/ so it worked fine after.
But I had to change my UPS after 1 year. Because it started inflating... That's my 2nd one now. Same model. So far so good after 2 years.