I'm basically reporting the same issue as discussed
here, which was said to be fixed in 3.1, but seems to remain.
As I sync files to my Android phone, the temp directory is getting increasingly filled up with .mp3 files (including those that *have* finished copying to the device), eventually filling up my system drive completely. They aren't cleaned up until I fully quit MusicBee.
I do have the "embed artwork in the music file" option selected.
MusicBee 3.2.6902.