That's exactly what the pin-pen merger is! As you know, it's not limited to pin/pen, and hearing ability (in my case, profound hearing loss) is not related to the ability to hear the difference. I don't understand the linguistics, but my very bad understanding is that there's actual brain chemistry here that means that you _can't_ hear the difference because you never learned it, never spoke it, and you pronounce them the same.

My partner is from the PNW and she pronounces "egg" as "ayg" (like "ayyyy-g") but when I say "egg" she can't hear the difference between what I'm saying and what she says. And she has perfect hearing. But she CAN hear the difference between "pin" and "pen", and she gets upset when i say them the same way. lol

But yeah, that's one of the things that makes accents accents. It's not just the sounds that come out of our mouths but the way we hear things, too. Kinda crazy. :)