Hi,
I haven’t found this bug (?) listed anywhere so i hope this is not a repeat.
When you keybind a spell to a mouse button like the middle mouse button you can’t use mouse over casting on unit frames. It works when hovering over player models.
You also don’t have that issue when the spell is keybound to a key on your keyboard.
I think this is a weird interaction with click casting. (Which i don’t use.)
Edit:
Ok i just tested it. If i configure the same spells i have on my action bars for click casting with the same keybinds they have on the action bar, then the system works. I think the problem is that the interface ignores all mouse keybinds when hovering over the interface. This is not very intuitive and annoying because i have to set up my mouse keybinds twice now.