That doesn’t mean he has to be demoted. He could take a leave of absence from Star Fleet to deal with his PTSD, during which time Mark Piper becomes the Enterprise’s CMO. Later Piper is replaced by McCoy. When M’Benga gets his mind right, he returns, but by this time Kirk’s in command and likes his command staff as they are, but offers M’Benga a post as general physician.

But if you insist on the demotion idea, he’s already done plenty to warrant it.

Keeping his daughter in the sickbay transporter buffer was a dubious practice, made suspect by him keeping it secret from command. His actions in Under the Cloak of War could get him a court martial, if they came to light.

There’s no canon basis to support this, just head canon inference. While this could be the direction SNW is going with this, being posted elsewhere and then returning for a brief period doesn’t imply any demotion.

M’Benga was working, likely shadowing, with McCoy in TOS for just an episode or two before he acted as CMO when McCoy was away. The fact that McCoy had to confirm/remind Kirk that M’Benga had trained on Vulcan strongly suggests he wasn’t posted longterm to the Enterprise. More, M’Benga was supposed to be on his way to head up a medical station, which suggests he was moving towards a position of responsibility.

Asking a former CMO to work alongside for a short period and then act when he’s between postings, especially one with specialized expertise needed for the crew’s complement, is fairly routine.