Old Ted Award
I notice it mostly with barrel aged spirits. Lots of tannins may be be the culprit.I suspect why there is a difference between adding alcohol based spirits directly and letting the vapors infuse the tobacco comes down to volatile organic compounds vs soluble organic compounds. By the infusion method only the volitile compounds are imparted to the tobacco. When added directly to the tobacco all the soluble compounds are also imparted to the tobacco. Some of these may be giving the off flavors and burn characteristics.
I remember when I was in college. I was already a pipe smoker and just starting with cigars. I decided it would be a good idea to literally soak a Henry clay cigar in rum for 24 hours and dry it out. That cigar tasted exactly like raw sweetness and nothing else. So whatever those compounds are, they effect differently depending on the way/amount they are applied and of course what level of tannins are in solution..