Thanks for the patch, Steven. Using this patch (3.0.5898), calling the now playing assistant seems to bring the entire MusicBee program to the foreground before opening the assistant. While this is better than not grabbing focus at all, it would be great if just the assistant window opened above whatever application the user was using, like in MB2.5. That way the user could get a new song playing/queued, dismiss the window, and get right back to whatever they were doing!
EDIT: Though the whole MusicBee application focuses, focus is correctly lost upon song selection, which is good. When focusing involves a Windows 10 desktop change, focus is not lost after song selection.