I can reproduce this here on Meta (not sure where you did the testing). I’d add that the Emoji picker postion isn’t quite right in any of the full screen editing cases.
i.e. fullscreen with preview it is floating in the middle of the screen
edit: As I was testing this I also noticed another quirk: If you open the emoji picker while composer is at full screen with preview, then exit full screen the emoji picker disappears. To get it back you have to to click the emoji icon twice to make it show up again. Any other combination the emoji picker moves aroun, but stays on screen