Is the spot you’re describing where bubbles form if you put soapy water on it? I just want to make sure it’s not leaking somewhere else. One full breath every 30 minutes is about 2 ml per second, so the bubbles should be fairly noticeable.
Would you be able to post a picture? You might be able to carefully shave off a bit of the melted plastic if it’s preventing the seal from closing, but it would help if I could see what it looks like.