2018 Mac mini shuts down despite UPS

I've been using my 2018 Mac mini (Ventura 13.0.1) with a UPS for almost five years and the behaviour was as expected -- power goes out, UPS takes over, Mac mini keeps running until I can shut it down. A few months ago, it started shutting down as soon as the power cut. Everything else shut down as well, so I figured the problem was the UPS battery. I replaced it, and I'm still getting the same behaviour with my mini, but not everything else that is plugged into it. I decided to try another UPS and same thing.

So the issue seems to be the Mac. It feels like something deliberate, like the Mac now is able to tell that it's on battery and not shorepower, and shuts itself down properly, because I never get a message at startup that it had an unexpected shutdown. It just reboots soon as power is restored to the UPS. My UPS isn't connected to my Mac to be feeding it any information.

This wouldn't be a huge issue if I was always work in Mac OS as the computer reboots quickly to put me back where I was at shutdown, but I'm frequently working in Windows (through Parallels) and Windows doesn't have the intelligence of MacOS to be able to recover from a power outage, so it doesn't autosave anything, loses settings on some of my software, and can't restart on its own, so it's a huge hassle to start up again.

Power is very flakey where I am and we have multiple micro-second outages a day during the summer, so I need to figure this out ASAP.

Thank you for any insight anyone can provide!