We are experiencing the same thing, it does not matter if we are using UseRHI or UseNativeDX12. We also noticed that frame interpolation sometimes does not work at all, despite starting standalone with the same config settings.
Have you managed to find the source of the artifacts?
Edit*
Appears to be r.FidelityFX.FI.ShowDebugTearLines. Set it to zero to get rid of it. Found it thanks to @nno_x in this other thread.