I think, especially with thicker liquids, this might be an issue of flavors "permeating" somehow throughout the juice. I might be wrong, but I have definitely observed that flavors alter a bit with age and this might explain it. (I had cut down enough on my smoking by the time I started vaping that I had a fairly robust sense of taste.) I haven't noticed the phenomenon so much with the handful of factory-made "commercial" liquids I've had.
With liquids that are mixed fresh (e.g. DIY or made-to-order flavors like TastyVapor) I notice that the flavors change over time, especially during the first week.... Someone pointed that out on the DIY forum and it explained why I noticed that immediately trying my DIY liquids didn't reflect how they'd taste a bit later.
Oxygenation could it part of it, no doubt, especially since some plastics are more air-permeable than others. (An interesting experiment to test this would be to have multiple bottles of identical-batch liquid in soft plastic, hard plastic, and glass bottles with varying levels of air in them, leaving them sit in a dark place, and seeing whether some change more than others in flavor if at all.)