You might have a sulfite allergy that’s triggering an immune response. Do you have any other skin conditions triggered by consuming food or drink preserved with sulphites or sulfides, e.g. red wine, dried fruits?

Nope, not at all. My skin has always been clear my whole life, and I often consume dried fruits (raisins are currently my favorite evening snack,) as well as many other commonly 'triggering' things (red wine among them, tea, alcohol and soda spring to mind, though I've cut back immensely on the last two)

That being said, you still might be right, since the science behind canker sores is a crapshoot to begin with, as I've learned

Personally, what works for me is: If I bite my lip, or feel a sore coming on in general, I immediately have to wash it out. After that, the only way to prevent a full week+ long sore is to put alum (pickling spice) on it for 90 seconds or so, once or twice a day. And of course, I need to make sure I meticulously brush and floss after every meal (of course with toothpaste without laurel sodium sulfate) Usually I can suffice for the normal once or twice a day

And that being said, this isn't exactly scientific. This is 100% just correlation and gathering personal anecdotal data. If I happen to stumble across something better, I'll take it. But all things considered, it's not a particularly bad thing to suddenly feel the need to take extra care of my mouth once a month or so