It's probably the PG. If 80% causes the problem but 50% does not...
Otherwise it is some throat hit additive that some vendors use but don't tell us about. For most juices, 12mg is 12mg and so on. For a few, 12mg seems to feel like 18, but its not a nicotine "thump". It's more of a "thump with residual burn" which amplifies the effect. Drip another juice on of the same pg% and nic and its back to 12mg feeling.
Try some nicoticket. Avoid indigo (although delicious for tobaccos, burns my throat quite a bit).