Simple: If the song features an extra artist. It should be written in the Artist field, it just makes sense. Even if official sources states it in the Title
Examples:
1: beatmapsets/126583#osu/322055
2: beatmapsets/743684#osu/1568261 did they forget to put (TV Size) in this map?
3: beatmapsets/1040690#osu/2176825
EDIT: I should have provided more recent examples so here: https://i.imgur.com/8uYlMDK.png
(they are different songs but still should apply anyway)
4: beatmapsets/1554093#osu/3175126
5: beatmapsets/1546547#osu/3172209
The first and third one should be correct however the second one has the featured artist in its title yet all three of them being the exact same song
should be written something like
Artist-A feat. Artist-B - Song
Artist-A feat. Artist-B, Artist-C, Artist-D, etc - Song
Examples:
1: beatmapsets/126583#osu/322055
2: beatmapsets/743684#osu/1568261 did they forget to put (TV Size) in this map?
3: beatmapsets/1040690#osu/2176825
EDIT: I should have provided more recent examples so here: https://i.imgur.com/8uYlMDK.png
(they are different songs but still should apply anyway)
4: beatmapsets/1554093#osu/3175126
5: beatmapsets/1546547#osu/3172209
The first and third one should be correct however the second one has the featured artist in its title yet all three of them being the exact same song
should be written something like
Artist-A feat. Artist-B - Song
Artist-A feat. Artist-B, Artist-C, Artist-D, etc - Song