This Picture is with Version 1.21.1-1.9.3

This picture is with Version 1.21.1-1.9.4

this is the latest Version and this is also bugged

Are you sure it doesn't happen in 1.9.3? Because the issue is a bit inconsistent in my tests.
If it isn't there in 1.9.3 it at least narrows it down somewhat.

mortuusars/ExposurePolaroid#9 - reported there as well

Indeed I tested yesterday every combination and only when I switch from version 1.9.3 to 1.9.4 this happens.i tested also the other stuff with Polaroid but only when I switch the exposure mod itself the bug happens.
I noticed it when I updated my mods and for no reason all players talk about this bug happens. So I narrowed it down to this.i switched every mod I updated and this is what it is.

Right now I use this modcombination on the server.

I have fixed this by force enabling direct capture when both Distant Horizons and Iris is installed. For some reason they don't want to work properly with background capture.
I would like for this to be a temporary fix, but since I lack knowledge of all these rendering nuances it'll probably be this way for long time, unless someone who knows better would help.