Why can't I AirPlay mirror my iPhone XS FaceTime video to my AppleTV in landscape rotation?
my wife's iphone xs is in the mail, so this is speculative, but i would guess the new resolution is causing an under/overscan issue to manifest. Try adjusting on the apple tv and see if it works.

On Apple TV 4K or Apple TV (4th generation), go to Settings > AirPlay > AirPlay Display Underscan.
On Apple TV (2nd or 3rd Generation), go to Settings > Audio &amp; Video > Adjust for AirPlay Overscan.