I'm not sure what setting is causing this. I have turned on/off every setting I can think to, and nothing seems to have helped. It's not constant, but a good amount of the time if I move my mouse, the sky/horizon flickers. Not to full solid black I don't think, and it's more prominent during sunset/sunrise.

I know it's an issue with at least Rubidium, as I disabled the mod and the issue went away. I don't know if it's a conflict with another mod I'm using or what, but I want to fix this soon as I have photosensitivity issues and this is starting to mess with me.

These are the other mods I'm using:
mod list.txt

Have you ever found what was the cause?

I have the same issue on 1.18.2

maybe this

I had this glitch without that mod.

Tried it with no mods besides Rubidium, still had the issue. However, after many hours of confusion and testing, I think I found the cause. Turns out, what I had done was used ATLauncher's "Use Minecraft Java" feature. The issue with this is, by default, Windows will let Java use only the integrated chip on your computer, in my case my Intel HD 530 card. The Intel card seems to not play nicely with Rubidium (maybe I just haven't updated my drivers for it) and caused the flickering. Forcing Java to use my Nvidia GTX 1060 fixed the issue and also provided a massive boost in performance.

For me, the fix was to make sure my launcher was using my Java path, and make sure that my javaw in that Java path has "High Power Mode" preference set in Windows graphics setting.