Voltage fluctuations on a 1000VA 600W UPS with 40% load when on battery

Hello guys,
I live in Europe (240v) and I've recently purchased a Line Interactive AVR 1000VA 600W UPS with in order to protect my new PS5 gaming console and TV. The PS5 (max 200w) and 42" LED TV (max 120W) are the only things connected to the UPS. If a video game is playing on console (to draw more power) and I turn off the circuit breaker for the power outlets in that room, the ups will switch to battery mode and the voltage will fluctuate rapidly between 190V - 280V. The load on the ups is at 40%
If I connect my PC (running a demanding task for more power draw) and monitor to the UPS and turn off the circuit breaker, the voltage is stable at 230v -235v, the load on the ups is at 40%.
I've replaced the UPS with a different model from the same brand with the same specs but better batteries and the problem is the same.

This has been driving me crazy for the past few days and I cannot get my head around it to figure out what is going on. I thought that buying a UPS will solve problems not add to them. I seldomly have power outages in my town so it was more of a safety measure.
Can you guys tell me what is going on? Was I just unlucky to get both UPS to be broken? Is there something else going on with the devices I connected to the UPS ?

TL;DR

UPS voltage fluctuations when on battery and at 40% load with only a gaming console and TV connected to it. The voltage is stable on battery, if connected to my gaming computer and monitor.

It may be down to how the PS5 power supply works, at a guess it's a switched mode supply. It also depends on the sine wave output type of the UPS.

A PC draws a lot more current than the PS5 and the UPS will be more stable at higher demand, can you add another load to the UPS when the PS5 is attached such as an incandescent light to see how the UPS acts? (ideally a 100 watt bulb).