I have exactly the same experience in Ubuntu Mate 19.10.
Since the external monitor is above the laptop monitor, and the primary monitor remains the laptop (bottom) monitor, whenever maximizing application windows on the laptop monitor, the title bar (including the window controls) disappear behind the menu bar.
I have exactly the same experience in Ubuntu Mate 19.10.
Since the external monitor is above the laptop monitor, and the primary monitor remains the laptop (bottom) monitor, whenever maximizing application windows on the laptop monitor, the title bar (including the window controls) disappear behind the menu bar.