The MaterialVariant property has an unusual mechanic in the property window where it won’t let you remove a MaterialVariant by simply clearing its contents, instead it absolutely requires that be selected as a choice (either written or selected by dropdown)
The whole experience in the properties panel for handling material variants is really clunky and uncomfortable. This is a huge part of the reason why. The other part is material variants’ reliance on hierarchy names as keys. You cannot rename a material variant without breaking your game, and you need to remember the material variant’s name and its parent material to assign it. Tooling side of this needs a lot of TLC. I don’t want to use a separate widget to change the material when I’m bulk changing many properties on a part or several.
It is really odd why they went for this design instead of Instance type like the Parent property. I see no up-side.
This will be fixed while rewriting the Properties pane to Lua (coming after the Next Gen Explorer).