This issue (or at least the flickering) is caused by an unflagged change that accidentally made it in a couple of weeks ago. We’re working on issuing a fix in a future release. (Edit: will be fixed next week)
My assumption is both these issues originate from a similar place.
I’m trying to reproduce the square brush issue and seem not to be able to on my end. Does this only happen with MaterialVariants? Does the square start at the center of the brush, or does it align with something else? What happens with larger brush sizes?