It is not so much the load it is because some power supplies need extremely clean power. The power that comes out of the UPS you have may not be true sign wave. I don't know the details and I am not sure how much is marketing lies but they say you should run modern computers on only sign wave ups.
What exact UPS do you have.
In general it should run your pc for a couple minutes even at 600watts. It is highly unlikely you are using that much power you would need to be running something like a overclocked 13900k and a 4090 to pull that kind of power.
This is one of those things that I never tried plugging my good machines into my old UPS. I have a better UPS that provides sign wave power for my gaming machine and I use the old UPS run run routers and switches and other random stuff.
I did my research on true sign wave and SSW and I got the almost the whole picture.
So to what I understand TSW always give me clean output power using it batteries if necessary even when my house power is poor , which I hardly believe my UPS one of them.
And the question that comes to my mind that my house power usually not good which makes my UPS go blinking and Turing green and red for hundred times in few minutes?does that make the issue?
And that goes me to another question
If that's true then why sometimes when my power are great and nothing wrong but then when it turns off my UPS immediately restart my PC? Can it be that when it's switch to battery mode does not give my PC the right amount of power in that sec cause it to restart? Or I opened my brain to wide😄?