Personally, if it was me, I would play safe and replace it. Something is going on there.
I would not even leave that in my house in case it ‘goes up’.
My wife had an iPhone 3GS battery fail. It just went pop as she was holding it and the display was pushed up. I literally just grabbed it off her and through it out of the house. It didn’t actually catch fire, but it swelled up like a ballon and wouldn’t chance it.