For episodes intended for television broadcast there is a difference between what’s on the film and what can be seen on most TV receivers. I’ve read that for TOS there are some scenes where you can see past the edges of what a TV could show and see edges of sets, lights, mike booms, etc. See Wikipedia https://en.wikipedia.org/wiki/Safe_area_(television)
I suspect that this is one of those scenes where they felt that it was not going to be noticed, either because the scene was so brief, or because your attention should be focused on the actors.
–Paul E Musselman