I run a two monitor jukebox and have no problems. I assume that is what you mean by movie monitor. A PC with two video outputs.
You set them as extended, and position them side by side with the "movie monitor" to the right of the main in the Video control panel.
For us, on first play, the video appears above Now Playing on the main monitor, you right click it and detach it. It will then fly over to monitor two and play fullscreen.
But if that fails and it doesn't fill right, we just mouse over and double click the screen to make it go "fullscreen". This does not work for you? The taskbar is still seen?
Do you have the main monitor set that way (1) in the Video panel? If the movie monitor is set to "1" it still works but we can find desktop items there, and opening a window, say the options page in MC, opens on the wrong monitor. Did you have to fix that once? Move something that opened on the wrong monitor? If so you worked around the problem, but not fully.
If it gets really messed up, change the video panel to "clone" or whatever so they show as one, then break them apart again. That seems to reset it.
Just some ideas.
Don