So: the fix in 21.0.37 works - thanks, Matt.
Here is the Expression for "Expression to group by:"
if(isequal([Comp], Various Artists,1), _Various Artist Compilations, ifelse(!isempty([Album Artist Sort]), [Album Artist Sort], !isempty([Artist Sort]), [Artist Sort], !isempty([Album Artist]), [Album Artist], 1, [Artist]))
I couldn't find other tags or library fields that indicated compilations, so I created my own user field [Comp]="No;Single Artist;Various Artists" If you don't have a Compilation indicator field, just eliminate the outer If statement.
For "Expression to display:"
if(isequal([Comp], Various Artists,1), _Various Artist Compilations, if(isempty[Album Artist, 1], [Artist, 1], [Album Artist, 1]))
Again: if you don't have a Compilation field, eliminate the outer If statement.
(The underscore in "_Various Artist Compilations" is so that it sorts to the front of the Artist list.)
Also: I have no idea why MC keeps putting the "1" in the [Album Artist, 1], [Artist, 1] field names - I didn't put them in and I'm not sure what they do.
Bill