OK. I guess the only way to put this to bed is for me to go through it in some detail.
I am only going to cover the case where you give the potion merchant the ingredients the first time, and get a single potion. You cannot buy in bulk until you have done this (including getting the first potion, so if you can buy in bulk, you got it).
When you give the potion merchant the ingredients, in the "Taste is everything" quest you get quest stage 30, which is "I have brought the ingredients for the potion." The dialogue where this happens is "Good. Give me a minute to prepare that antidote for you."
You can then move on to the dialogue "[Mixes the ingredients]", or you can leave. It doesn't matter, because quest stage 30 has been set, but quest stage 35 (more on that later) has not. So the next time you come back to the potion merchant you can pick up the dialogue again at "[Mixes the ingredients]". You can leave and come back to this as many times as you want, but since quest stage 30 has been set, but quest stage 35 has not, you can always come back to this point.
From there you can move to the dialogue "There. One potion against food-poisoning for you.". As soon as you do that, you get the antidote, and quest stage 35, "I received a potion of antidote, that should help me if I get food-poisoning.". Here's the code for the dialogue, with the key lines highlighted.
antifp.PNG
So first you get the quest stage, then, as fast as the code can execute, you get the antidote. Unless you manage to crash out of the game in the split second between those events, if you have that quest stage, you got the antidote.
Without quest stage 35, you cannot buy in bulk.
Hopefully that clears it up. There is no bug.