I've had a Format Values action that copies ALBUMARTIST to ARTIST for sometime now that works fine.
I also have a Format Values action that copies the 1st artist in the ARTIST field to ALBUMARTIST
I created a new action to format ARTIST with just the 1st value by cloning the 2nd action and changing the field to ARTIST.
For some reason this isn't working. I also tried the same format string to ARTISTSORT (successfully) and then using %artistsort% as the format string (fail). I tried again using Convert > Tag to Tag and the same format string. The preview shows correctly, it looks like the changes are being made and the results show that tags were formated but the tag is not actually changed. Oddly if I change the format string to $meta(artist,1) the value is updated successfully.
I also tested on another multi-value field and this only seems to occur with the ARTIST field.