sudo cset proc --move --pid=PID --threads --toset=user --force
Replace PID (e.g. 1197) with the current PID, and let me know how it goes.
That's the current workaround for now while I work on why it didn't move it before. I suspect this is a race condition (MPD didn't start quick enough), or I only moved the server app, but not the client app!
Have created a ticket, and I'll sort it out in the next release.