Im not sure if it is Vapers Tongue.
I have had cartos in a tank, maybe 2 re-fills of said tank and all of a sudden it begins to be a scratchy rough vape. I replace the carto but keep the tank filled and its gone.
In my experience, i have seen juice destroy a carto. For instance, Rip trippers Juice (which is awesome btw). he uses a natural sweetener and that will gunk up a carto 5-10 drags into it. I watched this happen on a few drippers that i have. 5 drags in the gunk buildup on the coil changes the flavor, TH and color of the juice. With regards to Rip's juices, i normally mix them myself with additional juices or bases. this cuts down his juice into something that can be vaped in a carto tank. His Nana's cream, my ADV, is mixed by me with 100% PG banana cream pie from MBV. this cuts the consistancy down a bit while keeping the flavor. After about 4 tank fills, its time to replace the carto.
All i am saying is, that it could be the juice reacting with the carto. at least that has been my experience with this type of issue.
.02$