Its likely because you are vaping cinnamaldehyde, which in high enough amounts will cause mouth sores. And if it is PG, then it is possible that the general drying effect of the PG is also making it worse. Many people, me included, cannot do much cinnamon, like gum or candy, because it makes the mouth break out. Cinnamaldehyde is also a lung irritant, as are many aldehydes. Some are fine with this cinn juice, but what you are describing is not at all uncommon, especially for cinnabomb. Do a forum search, and you will see.
In my professional opinion, as a chemist, vaping cinnamon is playing with fire. I would stay away from it, as it seems you are also one thhat gets cinnamon sores, and wash your mouth with a water solution of baking soda. This will raise the pH of the mouth and help a bit with the pain. They will go away, but only if you stop topically applying cinnamon oil and stay away from acidic drinks or food for a couple days. I'm not a doctor, so this is not medical advice, just what works in my experience.
I wonder if vendors are getting stories like this from customers that vaped their cinnamon juices. It would seem they would be. I have seen at least three stories like this for cinnabomb in the last couple months.
