I added https://beta.musicbrainz.org/artist/b1d3cda4-0cc3-4e5c-9e3a-35c8a2dbeab6 with the "subgroup of" relationship to https://beta.musicbrainz.org/artist/3cb86073-22d7-43d5-8f22-422b1e54988e but after it had been added, MB told me that
" This artist has no relationships, recordings, releases or release groups, and will be removed automatically in the next few days. If this is not intended, please add more data to this artist." So I went to check, and sure enough, the relationship isn’t there.

I then went to edit the artist to add it, but trying to submit the edit makes it tell me that "The data you have submitted does not make any changes to the data already present." – and the relationship is removed from the UI.