not to contradict anyone else's line of thought... this is from my own personal experience ... but I find that the flavors don't seem to be the cause of the "ickiness" after some aging.
The reason that I think this is because all the recipes I make on a regular/semi-regular basis are made from fairly large (120ml) flavor-only batches for me - banana creme, melona, Twinkie, starburst, green Apple, Apple Jacks. These sit for months at a time, sometimes, with no noticeable degradation in flavor when I whip up a fresh batch.
Thus I can only conclude that the particular flavors I use aren't the culprit and any degradation begins when the flavor recipe is mixed with vg, pg, or nic.
just my 2¢
You could be right of course but I have a feeling it could also be some, though not all flavours. For instance, a juice I make with dragonfruit and creams turns quite flat after some time. But then another which I make with english toffee and menthol has greatly improved over several weeks.