When trying to assist with subjects like this it would greatly help knowing what specific e-juice you are using... Brands, flavors, VG/PG ratio and nic strength. I think this is critical information to determine cause. I know for me... Cinnamon flavors cause vapor's tongue.
Another consideration... our mouths are full of natural occurring bacteria. This can be influenced by what you eat/drink and dental hygiene. I've wondered what effect switching to vaping can have on the normal bacteria living in our mouths. PG is used in hospital air systems to sanitize them and it could be effecting things in your mouth when vaping.
There's a whole world of cohabitation with bacteria and other stuff going on inside our bodies quietly all the time... Not my field of expertise but might be interesting to determine if vaping has an effect on this stuff...