It is the same with the other grass. Sometimes it even happens when trying to paint rock. Sometimes it won't. Making a map/terrain is just impossible at the moment. What a bad time to discover your interest in Arma modding :D

Exactly the same issue here. I performed the same troubleshooting steps.

No worries, let me know if you need more help. I got Workbench to work beautifully on Linux, it runs faster than on my Windows as well. Hopefully it only gets better.

Set your Proton prefix to the same one as the game you want recognized with STEAM_COMPAT_DATA_PATH="<your/path/to/prefix>". This way it works for Reforger. And in my experience, Enfusion workbench is more stable on Linux under Proton than it is on Windows.