If it’s a bug, then trying new tutorial will not help.
One way of testing if it’s a bug with the brush, is to clear your canvas and create new 3d mesh. Remember no mask on mesh. Shift + Crtl with ClipRec only effect mesh’s area outside of the white rectangle. Try Shift + Ctrl + Alt with ClipRec will conform geometry inside white rectangle into a rectangle geometry shape.
If it still not working then try out other clip brush like ClipCurve, ClipCircle, etc. If nothing works then most likely it’s a bug.
I guess to delete post, I ended up putting in delete post in title and body text and flag it.