When we first see Dr. M’Benga (chronologically), he is the Enterprise’s chief medical officer. When we see him again, he is still on the Enterprise, but working under Dr. McCoy’s command and, if he was on the ship at the time, under Dr. Piper’s command as well (Dr. Piper was CMO in Where No Man Has Gone Before).
So why do you think he was demoted to a secondary position? Was it something to do with his actions as the Butcher of J’Gal or his killing of Ruh? You would think he wouldn’t be on the Enterprise at all if so. Did Pike demote him? Did Kirk demote him? Was it on orders from Starfleet Command?
What do you think was the reason why McCoy became his superior officer?
Why do you think he was demoted?
This seems to be another example of something fans seem to view as ‘canon’ but isn’t actually consistent with what was onscreen. A kind of ‘Mandela-effect’ headcanon.
In TOS, M’Benga was specifically asked by McCoy to come aboard and act for him as CMO while he was away. Asking a former CMO to come back, shadow you for a short period and then act for you makes a lot of sense when there are members of the crew of other or mixed species.
It was quite clear that M’Benga was in transition towards a Chief Medical Officer post on a Station. That would suggest M’Benga already had the rank and was qualified for the job.
Meta-wise, this was actually another one of Roddenberry’s backdoor pilots, this time for a Starfleet medical show based on a station.