To me it looks as though you write the whole artist field when you edit any of the columns - you do not address one of the artist multi-value fields.
I would have assumed that you had to use the same expression for field as for value, but I am not even sure that that would work.
I'm sure it used to work - what happened occasionally was that the separates needed updating so in artist 1 it would read "Bob//Harry//Jim" all together. What you could do is click in the artist 1 field, press return and they would separate out it the correct columns, but now they get deleted.