I'm not currently using 899 (downgraded for now to 893 for the multiple tracks issue) but during my brief spell with it I noticed a similar problem, as the splitting (of Album Artists, at least) didn't seem to work as it should. Interestingly, I can see the reason for this even in 893 (which doesn't have the splitting option, of course): My multiple Album Artist tags are split by a semicolon, so e.g. I have Beatles, The; Lennon, John; McCartney, Paul; Harrison, George; Starr, Ringo [the idea being that the Beatles albums show up along with the solo albums from every member, so selecting Lennon, John would show his solo work and the Beatles albums; this works a charm in MusicBee]; the bold printed tag was copy/pasted from TagScanner. The semicolon option (without a gap) seems to be pretty standard, MP3Tag does it like that, MusicBee correctly splits them up accordingly.
Now here's the weird thing: When I copy the files onto my MP3 player (Fiio M9), using MusicBee, Poweramp (893) displays these Album Artists like this Beatles, The Lennon, John McCartney, Paul Harrison, George Starr, Ringo - yes the semicolon is removed! Small wonder the splitting doesn't work, with the splitting character being dropped! Apparently, this seems to be an issue with Poweramp - I'd considered an issue with the synchronising software (MusicBee) as well, but when I copy the files from the MP3 player back to my computer and analyse them with TagScanner, everything looks as it should.