Not sure, it could be that you are just getting so used to the flavor to the point that it no longer registers with the taste buds. If you have a different flavor try it and see if it too dissapears.
I don't think this is related to your inquiry, but if the tanks are overfilled, they tend not to siphon properly, which can cause a burnt taste, apparently it needs a little air in the tank. For this, it is often recommended only to fill the tanks about 3/4 full.
If a burnt taste is noticed, which is indicative of an atomizer running dry; and the tank still has liquid, I have found that by taking the tank off the atomizer, then blowing directly in to the atomizer before reattaching the tank has helped it to regain the siphoning effect.