Recently I kept facing the same problem. It doesn't matter which brush I use, there are these weird shapes appearing.
I to smooth it out but it just remains there.
The above image, is where I tried to smooth it out, but as you can see it only smooths the surrounding.
The above image, is where I use the draw brush to draw on it. A large scratch mark appears.
This is one of many, that kept appearing. I tried to delete the part with such problem and replace it with "fill hole. However, it didn't work, as it appears in another place (I did some testing, it seems to only appear in a specific region, for my case it between my model nose and chin.)
Other details : I have dynatopo to the sculpt
setting resolution: 90 - 100 refine method : subdivide collapse Detailing : constant detail smooth shading : unticked
Expert guidance is very appreciated