apologies. a reboot fix it.
Split screen Mail.app has one major issue in that on an external display when the overlay window is down the bottom of the screen then its ghost appears on my MBP's non fullscreen space.
Somebody didn't do their QA properly, Apple.
I don't seem to have this issue. Can you post a screenshot?
My laptop screen is diagonally below the external screen instead the display screen layout. Try that.