APC UPS switches to battery 50 times a day

Hopefully this is the right place to ask.

I have an APC Back-UPS XS 1400U that I use to keep my home server running 24/7.

It was purchased in 2015, batteries replaced around 2020, everything was fine until around June 2023 when it started randomly switching to battery for a few seconds for no apparent reason once or twice a day.

The UPS is connected to my home server via USB so I can get some readouts. It says "Unacceptable line voltage changes", but it's configured to switch when it's outside the 160-280v range and it gets nowhere near those thresholds, the voltage fluctuates in the 224-234 range.

I connected an oscilloscope to the mains to see if there were transients when the problem occurred but I don't see anything out of the ordinary and the problem has been getting worse, now it switches an average of 50 times a day.

The UPS still works, it can keep the server up for hours if I unplug the power, so the batteries should be good. What's going on?