*sigh*
What's going in is that you have crappy liquid with too much sweetener in it. The heat from the coil is caramelizing it, thus the change in colour and flavour.
It's not the VG/PG, it's not oxidation.
It's simply the liquid getting too hot and doing this due to the design of the tank/coils, which results in eventually gunking up your coil/wick, and burning them bit by bit, giving you that delightful change in flavour. Trouble is, once it starts (this caramelization and gunking) it gets worse fairly fast.
Note that the Nautilus tank is really bad for this simply because, even though it's a well liked tank for some reason that boggles me, in my opinion it's a crappy tank. My wife and daughter got some because the store recommended them so highly. I never liked them, and part of it was because they would do this to a lot of liquids (which meant a lot of emptying of tanks and coils having to be changed). I stuck with other tanks while they used theirs as they didn't want to "waste" the purchase of it, until I got into sub-ohming, which once they tried my new tank/kit (at the time), they both switched and the Nautilus tanks went into a drawer and it's never seen the light of day since.
Depending on what you had as previous tanks, you probably had some that did the same issue with too much heat transferring to the liquids you're using, simply bad heat dissipation.
Since getting better tanks, I've never seen this happen again, and that's including with tanks that can get so hot that you could burn yourself on them, but that's because the heat is being pulled out via the metal instead of using the liquid as the "coolant" as per what some tanks like the Nautilus does, which it terrible for it.
