Think it's not mandatory to put the artist names even for singles although loads of anime related singles have made it look like it is. The tooltip part from edit mode:
Quote:
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A slash (/) may also be used at the submitter's discretion to include the artist's name on an artist album, as Title / Artist.
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I mean it says 'discretion' so that would be the user is neither right or wrong if he/she includes artist name or not.