I’m using the grab brush there and the only modifier is the Armature. This seems to mostly happens on posed areas, not all and there are 2 small areas where this happens even when not posed. I have never seen this before, I’ve been sculpting posed characters for 5 years. It looks like they are being moved with individual origins and by normal axis or something.
Update: It’s caused by the bendy bones but I dunno how to fix it… Once I set segments to 0 the issue went away
Update: A workaround https://extensions.blender.org/add-ons/pose-shape-keys/
Another update: It’s actually a bug https://projects.blender.org/blender/blender/issues/150080
I had this kind of problem before. On Armature modifier, clicking 
on “Preserve Volume” helped me.
Maybe Grab Brush’s settings could be damaged. Can you reset the brush? On side, took section, right side of brush image, you see a down arrow, click and choose Reset Brush. Maybe that works. Because your brush effects the inside normals too
Alaska confirmed the bug report, all I gotta do now is wait
https://projects.blender.org/blender/blender/issues/150080
I will edit the original post with a workaround, thanks for reminding me.
It can happen to meshes with modifiers applied. The modifier was put in place with the mesh in one position, then the sculpting changes the mesh but the modifier doesn’t “know” what to do because the positions of the vertices changed.
Temporarily disable or remove the modifier, make the change to your mesh then re-apply or re-enable the modifier. Now that the modifier “knows” what the mesh is, it will be able to make the required deformations without making your mesh go all catty wompass.
I should’ve added the bug report in the updates 
That’s not the issue tho, because it happens on a brand new blend file, in a brand new cube with just an armature modifier