Same issue for me in Mesh Paint Mode (version 5.2.1). Landscape Mode works for me.
I’ve tried all the things I’ve read about and could think of, but no resolve.
Looks like this is elsewhere too. Would be nice if this workflow issue could find a good resolve:
(UE5) Mesh paint Brush Size keyboard shortcut error, related technology additional request - General / Feedback & Requests - Epic Developer Community Forums (unrealengine.com)