I know someone from the Aberdeen area who says it that way, and I swear I've heard people in Spokane do it too, so it's not just Seattle at least. Though it's very possible the Spokane people weren't native to Spokane, I didn't ask.

My question for you, is do you notice it when eastern WA people say it? Does it stand out to your ear? If not it's possible that you're just used to hearing it either way and not attuned to the difference. I'm like that when people say "pen" and "pin" differently, I can't hear a difference at all. But the "bag" thing stands out a lot since it's different to me.