I have just realised a fundamental issue with Compilations...

Apple Music has a single user-facing tag for compilations.

So "Hey, 90s Rock Songs" is a compilation but so is "The Best of Beyonce".

But you want one to appear under the artist in your library - Beyonce - and you want the Dad Rock compilation to appear in compilations.

But with only one user facing tag, it is all bundled in together, hence the complete mishmash of compilations, mixing true compilations of multiple artists and "best ofs" from single artists.