Do you have translucent mode enabled? That’s what did it for me. I turned that off and everything got back to normal, but for some reason while it’s turned on it’s super laggy for me.
I have a KeepNote repository which was converted over to MD files; it’s huge. I don’t see any slowness, or failure to paint. Running in a Linux Mint Debian VM. It is good you captured it in a pic. No picture, it didn’t happen.