I've been having this on and off for two weeks. I'm pretty sure I've figured out the cause for me: the wicking in the atomizer and juice types.
I removed the wicking and bridge in the atomizer, and the taste/flavor started coming back within a couple hours. I think the wicking gets "Gunk" as other people have suggested and this gunk *re-burns* as you heat it. Imagine a cooking pan with oil that is never cleaned, the oil hardens, blackens, then imparts a burnt/acid taste to the food. I believe this process actually creates acidic pH levels that in turn cause mild acid burns in the mouth. I used to fight forest fires and breathing that smoke or even steam would do the same thing. Search Youtube for videos on how to "de-wick" the atomizer and remove the bridge if you drip (not with cartos).
Also, some juice does this, not sure if it's the company, or the PG, or the flavor, but two vendors (Virgin Vapers, Aroma ejuice) never cause a problem for me, so I'm sticking with them after much searching. There might be others vendors, but I think people that use higher quality or home-extracted flavors might be the key given that so many other vendor flavors cause trouble for me.
Take probiotics to restore natural flora in mouth, drink Kefir, eat yogurt, and take a tums before you sleep. When you wake up, it should be gone. I had it for 3 days, removed the wick/bridge and watched what juice I used and I was fine later that evening.