I've had 2 episodes of severe heartburn.
From previous visits to a DR (years ago while smoking analogs)- he said that among other things- Nicotine relaxes the flap at the bottom of you esophagus, allowing stomach acid to travel upward: which is heartburn. (And it is noticeably worse when you lie down.)
I have been taking a daily 'acid reducer' for years, but the two episodes occurred despite that.
I am a DIY juice person, I usually vape 18-20mg during the day. I recently made some 10mg juice and I have not had the problem. BUT, time will tell.
The comment about a small amount of e liquid being swallowed is also interesting, as there have been occasions where that has happened; I'll have to keep an eye out if that causes the heartburn.