This is a very simple, but extremely annoying inconsistency that has existed for way too long.
The rig builder in the Avatar tab of Studio simply ignores the concept of MeshPart heads. Since MeshPart Heads were enabled as Default, it has remained using part heads - meaning using dummy rigs as the base for… well, anything, is inefficient/impossible. I’ve resorted to using free-models or ripping my character’s items off and copying my in-game avatar instead because of this inconsistency.
What’s worse is that Load Appearance for Humanoids is annoyingly backwards compatible, and loading a new appearance on a humanoid doesn’t over-ride the head that roblox Studio forces developers to use.
This oversight is hopefully easy enough to fix and I honestly am kind of embarrassed it’s been in for so long, so if it could be patched right up that’d be LOVELY.
Now if you excuse me, I’ve got to go annoyingly get a real dummy humanoid into my game the old fashioned ways…