Ok, I know this thread is ancient but I found it helpful. I too have experienced some of this on the inner aspect of my lips at times and have wondered what the culprit "really" is.
I think the most likely is the reference to certain older stems that have oxidized, or are prone to continued degradation. If so, that is good info for me to know.
The last time I noticed it was with an older pipe I'd only recently acquired, and this stem could be the culprit.
I was wanting to get someone to make a new acrylic stem for it anyway...
Thanks all, I'll be paying more attention to this now.