To reproduce:
1. Open any project with set up Polybrush included
2. While tool is enabled and cursor hovering over paintable surface click once and don't move the cursor
3. Click and hold right mouse button, while doing so, move cursor away from hovering the surface being painted

Expected: no visual changes
Actual: painted vertices material is no longer rendered

Note: can also be reproduced by entering and exiting play mode, while not hovering on painted surface afterwards