Hello,
I don't know what triggers it, but the toolbar at the top of the MB player (where all my custom buttons are) has a small white line above each toolbar button to visually indicate it.
The odd thing is, whenever I move/resize the window, they disappear. Tbh, I don't even know why they're there in the first place, but I like them. Hoping to find out what's making them disappear so I can keep them.
Screenshot attached. It's hard to see, but there is a little white line above the text of each button.