Two Monitors, A Full Screen Video, verify—
Navigate to > System Preferences > Mission Control and check if Displays have separate Spaces is selected.
-----------------------------
Mirror 2 of 3 displays
Click the ? at the lower-right of the "Arrangement" tab of "Displays" system preference.
"To set up a combination of displays showing mirrored and extended desktop images, set up the three displays as an extended desktop, and then Option-drag a display icon onto another display icon to mirror the two displays."
--------------------------------
Try using extended desktop mode instead of mirror display mode. Mirror display also mirrors the display resolution. Extended desktop mode allows you to select the native resolution of the external display
Use external monitors with your Mac - Apple Support
https://discussions.apple.com/thread/250077037