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 my mates s 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.