@Duimon & @hunterk I did some further testing and was able to get a fresh retroarch install to not produce moire again (don’t know why I couldn’t achieve this when In wrote my earlier post).
So here is what I did:
I redownloaded Retroarche version 1.16.0, just to be sure and set it up from scratch again. I did not update the shaders within Retroarch.
There was no moirde. For some reason I cannot take screenshots from the parameter settings of the shader, so I’ll have to explain:
The interlace setting is set to 4 by default (whatever that means):
After that, I updated the slang shaders in retroarch and then there was moire.
If I set the interlacing mode to -1 however, the moire is gone (because there are so scanlines, I guess).
Conclusion: I think with the non-updated shaders, interlacing mode 4 did not show any scanlines, for some reason. Was this a bug in the GUEST shader that was fixed in a newer version, so now scanlines DO actually show?