On Dual Monitor setup, Office 365 is afflicted by a known bug n the main display; many users are experiencing problems with Office 365.

The top of the window (the first inch) is missing... the title bar the close minimize buttons are not there and if there a window underneat it it will close that window instead.
It happens randomly with many applications like file explorer, google chrome, adobe reader dc but always when Office365 is running.

All those apps are up to date, Windows10 is also up to date... the only way to reset to normal conditions is to reboot.

Minimizing (with Alt+Spacebar) allow to work on other applications but the reboot is not a solution...