I do not think the surface condition of that stopper is a result of some BB or other solvent.
It most likely was that way to begin with, before even being on that vial -- my guess is that this rough surface was a result of heat or the stopper production itself or something during sterilization steps etc.
Think about it, if it was from contact with a solvent, then the entire internal side would be much worse OR even completely dissolved by now.
It does not make any logical sense that a small drop of oil gets on the top can do that, but the entire stopper on the inside is still there while being in contact with the same oil throughout shipping and maybe even for over year since production etc.
If it was from the carrier oil formula, the other stoppers would also all show this because they all have the same exposure to the carrier oil formula.
You can drop some of the carrier oil on the top of a stopper and let it sit for many days, my bet is you will not see anything happen.
That is why my guess is just that that specific stopper (and I have seen a few others like it) started out that way from heat during production, sterilization, or simply because the stopper source, like any business or producer, has some outliers in its own production line.
No need to throw it away unless you know there is a leak (from the pic, i do not see any signs of leaking). It may look different, but functionally it is the same.
If you are concerned about the seal, simply push some pressure into the vial, a couple cc of just air pressure, then flip the vial over so the oil is in contact with the edges of the stopper; let it sit this way for a day or so with all the air pressure pushing the oil against the seal.
IF there are any leaks, the oil will come out under this pressure (really, in practice we would not even have this kinda of pressure, so if there are no leaks with it like this, you can be confident there are no leaks under normal conditions).
If Frank or his assistant know more, maybe they can add something.
but those are my thoughts and I hope it helps