They do explain this one, that the infusion from Spock bled over some of his logical characteristics.
Now does it actually make sense....no it does not, but its the kind of hand wave Technobabble that is perfectly established on Star Trek, so I am fine with it. Its exactly what I needed, just a quick explanation that fits the narrative of the universe established, and then we move on.
The episode was funny and interesting, though I agree with several here this season definately feels the weakest of the three seasons overall. Lower budget, goofiness and gimmicks over substance....extremely quick pacing to blow through plots without getting a chance to breath in the drama and consequences. But I did still enjoy this one.
My one beef with it is....they suggested the notion that the 4 crewmembers could actually choose to remain Vulcan, and there was the possibility they had to let them do that. That....would be illegal by Federation law, you cannot do genetic engineering, and if turning someone into another race is not genetic engineering I don't know what is. Could you imagine if they just let Federation citizens "choose to switch races"?
I can respect the idea of genetic alteration as a temporary measure, we have seen its use in TNG for example (though again this episode goes to that WAY WAY WAY too quick. Ok we have this mission, these people only know vulcans....I know lets shoot up 4 of our crewmembers with an experimental serum that will literally rewrite their dna....should be fine. That's the pacing problem I've had all season). But at the end of the day there should be no world in which the doctor is authorized to leave genetic alterations intact if they can be reversed.