Okay so I did some diagnostics thinking it might be one of the upscalers (TSR, TAAU, DLSS, XeSS or FSR) and it’s FSR. Somehow the game defaults to it (altho default was TSR) and switching to any of the other 4 or even switching to FXAA/MSAA removes that flickering instantly.
Edit - it seems to be the RHI or the D3D12 Backend from FSR3. It was enabled in project settings but FSR itself was disabled. The command was FSR3 OverrideSwapChain I guess. Switching the backend OFF but letting the RHI be ON somehow fixed it.