I had to replace battery recently on APC UPS and it didn’t lose power. But did make a continuous alert tone that was so annoying that turned off UPS and pulled it out.
That's just not true.
on shitty ones maybe, decent ones with bypass not so much
I have one of those absolutely terrible cheap Cyberpower ones, and I actually use it with the battery completely removed for some weird reasons. Works fine, though it gets upset and wants to sound its buzzer even with that disabled in its configuration utility. A few dabs with the desoldering tool fixed that.
But really, don't buy those things they are awful as a UPS. Never did work reliably.
Shitty UPSes are the ones which does NOT ALLOW you to replace rhe battery in-situ while critical load is being applied without disruption of critical load.