Originally Posted by LilRaverBoi

Well, my thought is it might be caused by ants. I really can't prove this and don't have evidence to back it up. However, I do know that ants secrete formic acid when they bite things. Formic acid is known to cause chemical burns on eyes/skin of humans/animals, so I don't think it is a stretch to think it could also damage plant tissue. Plus the pattern of it along the midrib would make sense for the path ants might take while traversing a banana plant.

Again, no conclusive evidence this causes what your pictures show, but it's a possibility that seems to make sense to me, anyway. And you DID find ants in the package, so that kinda helps solidify my argument.