If you are pointing the stem toward your tongue while lighting, that might be doing it. Happens to me once in a while and then realized what was happening. Noticed I was tilting the bowl up a bit to see where the flame was going and the mouthpiece was pointed straight for my tongue
Not really sure what's causing it to be honest - the stem is definitely the right way round! It is an estate Peterson, and I just noticed today that the hole is drilled to high so it's a pretty bad smoker anyway. If my conscience allows, I may stick it on ebay, or it may just join the collection of pipes I have that are too bad to sell on.
You may be able to file the draft hole downward. I adjusted a few that were drilled too low, believe it or not. The bit did not exit straight, but left a long section of partial exposure I had to file out. I used a drill bit by hand going backwards to help guide the draft hole as a start. (Needle) File work helped after that. I also used it in the shank/stem mount of another that would not pass a PC and had a very bad draw, with lots of gurgling. If you really are putting the pipe up as a non-sellable item, it is a perfect candidate for such mad scientist-y attempts. If not, perhaps trade for another flawed pipe you believe is more up your alley to fix. I set one aside that I did not believe I would ever repair, but it was quite unique (Use PSF search engine for Radiator Pipe, or message me for a picture.) With some practice on fill work, it looks (overlooking one spot) new! My point from this tangent is that you may find yourself able to fix the pipe later on. There are stem conversions, and people who do the conversions for a fee, and with some file work and pipe mud the pipe may be a good (or great) smoker later on. A person is often hardest on themself, and does not give credit to their own skill level. Research some and give it a go, the worst that can happen is you fix it, and start fixing others, and get hooked on that as well as purchasing pipes and tobacco... Or, it becomes a pipe that looks nice and sits on a rack.