It seems (from practical experience
) that some settings are only saved when MusicBee exits.
I have recently encountered this issue both with Customise Toolbar settings and the Playing Track - Displayed Tabs settings. It may also apply to other settings as well. I found that after a MusicBee crash my latest changes had been lost, and the settings had reverted to the previous state as far as I could tell.
So if you're working on settings such as these, do yourself a favour and exit MusicBee occasionally to save your updates in the appropriate configuration files to protect yourself in the (very rare) event of a MusicBee crash.
I've made a whole lot of assumptions here, and I was unfortunate that I'd made lots of changes during a single MusicBee session. I'd be interested to hear of others' experiences, and also if my underlying assumptions are a bit wide of the mark.