On the white part of the skybox, every time the heartbeat effect occurs it flashes black. Heres a screencap i was able to get of the frames where it does that

I don't really know how to fix this

Any other mods installed? Maybe it's a conflict with another mod? In theory I do not support 1.18.2 anymore. Would be interesting to know if this still happens in 1.20.

I will close this issue for now. Could not reproduce it. If it is still there in 1.20, post a list of mods and I will reopen the issue.