I investigated the MusicKit JS code and found a workaround!
For anyone wondering: the internal function playMediaItem checks not only the property previewOnly but also if the media item has a previewURL. When I pass in the ID of a library song (e.g. 'i.eEmBTAWEMr9'), this check fails. When I pass in a catalog ID obtained from song.attributes.playParams.catalogId (e.g. '1313047951') the check completes and the player correctly plays just the preview.
This is still unexpected behavior and I would expect this either to be mentioned in the documentation or to be changed in MusicKit so the player checks for the catalogId itself.
Adrian Freakout
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Very confused right now. I have the same problem because I want users to be able to reorder elements and this is nowhere in the documentation.