They market it as a pure sine power supply: https://eu.ecoflow.com/products/river-portable-power-station...

Is there another appliance that I can test to see if that's the issue? I don't have an oscilloscope.

Edit: I just tested powering it from a UPS: It's flickering more now and when it show the correct content it stays for a second more then when powering from the ecoflow.

The monitor model is: BenQ GW2780-T.