I dont know if any of you have been getting this but from what I’ve seen its everyone. From the recent update, on both servers I have, the material tool will only set the material and not right click it back, meaning once its set it cant revert back to the default. So I went around and it wasnt just my servers. I took a look and made a very simple edit and it now works.
In your gamemodes/sandbox/entities/weapons/gmod_tool/stools/material.lua on the server files, add in at line 98:
list.Add( "OverrideMaterials", "" )
Hope it helps