It seems that, under certain circumstances, a user can see Black Pixels while having a Filmgrain PP Effect active.
It appears that these black pixels only appear when sharpness is set to 0. Switching it to 0.001 resolved the issue.
I tried to reproduce the issue on my Nvidea GPU System - i also adapted all of my Video Settings to match his(besides resolution) but was not able to reproduce, but we have been able to reproduce the issue on his system, see Steps to Reproduce.
We're unsure what might be the cause, but we noticed that he has an AMD GPU while i have an Nvidea one. According to his experience in the past with drivers, it very well might be related to that.