It seems some people pick this up from certain cream flavorings, i have heard of many people stating that VBIC (TPA) has a peppery taste over 3%. Since a couple of them are TPA one could hit the components list and narrow down the chemical that causes it. A lot of people move away from TPA as their DIY progresses but i still oft use them for this very reason, they are the most transparent company working with the community.
I pretty much only vape custards and cure them all for two months before i touch them. When i find something i like i will make 240ml unless i really like it then it is either 500ml or 1 liter bottle. For my main ADV there are two litters on the shelf, one i am vaping from and the other is curing. When i get close to finishing the vaping liter i make another for curing. I also make flavor bases of my ADVs and let them cure for awhile before i use them. I have a love/hate relationship with time, i hate the wait but i LOVE the results..
One of my absolute favorite combinations of flavorings/extracts is Vanilla (MF) and VC V1 (CAP). As it cures the vanilla develops very nicely, rich natural vanilla flavor that adds a bit of creaminess to the mix, .5% was my sweet spot with it, of course that is just me as someone else may like it at higher or lower %'s.
Another is Cheesecake (LA), adds a bit of sweetness and mouth feel. I keep this at 2% or less.
Now im just rambling, ill stop. hehe