A professor, whom you had to agree with or feign agreement with to get course credit, and still have some contact with, told you, a vocal, happy vegetarian, that he was also a vegetarian

He was a vegetarian first. I became a vegetarian partly out of respect for his reasoning. I also didn't mention to him that I had become a vegetarian until I'd been one for, IIRC, almost a year and a half.

"Wow, this noble gentleman tried to reduce animal suffering by being a vegetarian until -- darnedest thing! -- he simultaneously developed over six food allergies, which makes him now a non-bad-guy omnivore."

It wasn't simultaneous. I think he was lactose intolerant all along, and developed the soy and legume allergies through overusing those foods; I'm not sure when the eggs came in. Eventually a doctor advised him to reintroduce meat to his diet.

Now, I don't mean to offend, but what made you reject the shorter hypothesis of "He lied to you, then covered it up"?

I've been to his house and had a meal there and there was no sign of meat in the house. I've been to restaurants with him and he flipped straight to the vegetarian section. I've met his wife, who is Indian and (I believe to this day) also a vegetarian. I've found him generally trustworthy. He had no reason to falsely claim to be a vegetarian the first time I heard him say it, and made no effort to conceal the fact that he ordered a dish with pork in it when we went to lunch after his return to omnivorism.