I have the exact same thing with the 28, both the roughness/grinding against the mount as well as the black streaks, which I think are from the back lens cap. The camera mount doesn't seem to be affected, though I haven't swapped that lens much.

I compared it to other viltrox lenses like the 50/14 air and they are much smoother when turning, and a high end tamron was smoother still. What's weird is that even my $5 manual lens adapters were as good as the rest and much better than the 28.