I noticed this today too. If I click a song rom an album, it adds that song and the rest from the album to the queue. If I then add more via "add to up next," it adds them after the current playing song. It has to be a bug.
However, if instead of selecting a song to play and It automatically adding the rest of the songs in the list, you select the whole album (or playlist or whatever) and do add to up next (with an empty queue), the next album (or group of songs or playlist) you choose add to up next, it does it properly.
My only guess is since you aren't technically choosing the whole album in the first instance, it only treats the song you selected as the true "queue" (even though it added the rest of the album for you).
Either way, it's either or bug or a feature that is dumb. Add to queue should be just that.
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