I spoke with the owner of a popular company yesterday, who sells flavorings from many different manufacturers. After discussing a few flavorings I had some questions about...I asked her about a DIY
eliquid I'd made about 6 months ago. I just recently found I had about 20mL left of it. It was great when I'd first made it and used the other 30mL from the bottle. But now the 20mL tastes alittle "soapy" to me.
She said flavorings for eliquids don't "go bad" per se, as in spoiling. If they do change individually
at all over about a year's time or more, it's usually just a very slight weakening of the flavoring. However,
some flavoring combinations reach a point, long after thoroughly blending, where they begin to develop a taste that's less than desirable. Said if it's a "soapy" taste, it usually happens more frequently when there's a higher total flavoring % in the mix.
So, based on the above, maybe the "bad" 1 of your eliquids had quite a bit more flavoring in it, than the other one. However, I've also had some small 15mL bottles of tobacco flavored eliquids (from 3 different vendors) that
all went to being "funky tasting & bad smelling" after about 6-7 months. They were in glass bottles, kept in a dark cabinet, in my air-conditioned house. So with them, it wasn't because of sunlight, nor oxygen exposure, nor being in plastic, nor because of heat. I have no idea why
they all went "bad".