Adding to my last answer: After Covid I've washed the area with alcohol multiple times, and I think that could be another reason the natural defense of that area of my skin is weakened.
I had the same issue with my hairs for many years, the thing is that I care so much about my hygiene, I mean more than ordinary people, so I used to wash my hair with shampoos multiple times in a single shower, and I used to go to shower for long times and almost every one or 2 days. Anyway I managed to get rid of it after not taking showers frequently. I started with not taking shower for 4 to 7 days. Then I started taking showers every 3-5 days and everything is fine with my hair. Also since my hair used to smell I used to put my hand in may hair and smell it which wasn't a healthy habit. Once I realized that the area under my computer's desktop smells the same way. My guess is that the reason was that the area is dark and I didn't clean it so much since it was under my keyboard anyway. these days I clean it with Alcohol regularly. The point is that we've kinda destroyed our natural skin defense by washing it a lot, you know that there are enzymes in our skin sweat that fight microbes. Honestly it's a few weeks that I have the smell issue with the area above my chin and below my lips, I'm kinda sure it's the same bacteria, fungus, etc. I've realized that I touch this area regularly, and I'm going to stop touching it and stop taking showers and washing the area frequently for a while.
Have you ever had eczema? It's kind of on a continuum with allergies and similar. Maybe what you have is a form of that.
There aren't any experts here, just folks, but we do have experiences. First, being vegan doesn't mean you're at all health conscious. Vegans are like everyone else, some do it in a balanced healthful way and some eat lousy food and do it just for ideological reasons. Being vegan is very hard to do in a healthful way because there's no complete protein and no B12, so you have to consider that. It's hard to do right. And here's the thing about antibiotics -- they giveth and they taketh away. The smell issue sounds more fungal than bacterial, but if antibiotics made it go away that does say something. But antibiotics also kill off a lot of what makes us healthy, especially in our digestive tract and in our immune system, and so they can almost guarantee something you don't want is going to come back in a worse way. That's why we aren't supposed to use them often. Fungal infections are especially prone to coming back because the antibiotics often don't kill off the eggs. Now, that being said, that isn't an area prone to fungal infections, and smell is usually associated with yeast. Is your tongue white? It could be thrush, though if it were your doctors should have been able to see it if they looked. The other possibility is that you got a placebo reaction from the antibiotic which would make it an emotional thing. Obviously, can't diagnose you but when you think something's wrong and docs don't find it, find better docs.