Considering that M’Benga is demoted, it’s likely he did something that is consider attrocious, but not to drum out of Starfleet entirely.
Considering the Cliffhanger, what if M’Benga developed some kind of permannent genetic engineering that changed Gorn from reproducing with Paralystic Eggs being pregnant with their own paralystic eggs?
The reasoning I say is this: with Paralystic Eggs, they are consider direct threat to well being of the various Species within Federation due to nature. However, in Kelvinverse Gorn can get pregnant; and by the time Lower Decks come about Gorn is “friendly enough” for them to do business within Federation, something may have changed - voluntary or not voluntary. I will say not voluntary.
The technology exist: Klingon’s smooth forehead is a combination of genetic manipulation with virus, leading to the spread of the geneitic engineering beyong those in the experiment.
Now, if M’Benga did it, it actually put Starfleet in a dilemma:
- On one hand, M’Benga commited permanent genetic engineering without authorization, as well as cultural/genetic genocide. Grave crime in Federation
- On the other hand, it’s Gorn, who attacked Federation space and colonies.
Thus, the demotion - if it’s just the first part, he will be dishonorably discharged. But since under the table he effectively saved Federation, they decided to just demote him if he choose to stay on Enterprise, or assign to some random outpost. He chooses the former.
What makes you think that M’Benga will be demoted? There wasn’t anything hinted like that at all
He stepped down from CMO by 2265, and served under McCoy as “general physician”
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.