This happened to me quite a few years ago. I heard that someone in the band I was in kept playing a wrong note. After a while I realised it was me, on my 56 key Wheatstone bass/baritone!
I checked I was pressing the correct button: yes. The concertina is single-acting, so I couldn't even play that note in the other direction.
At the end of that number, I played a long note and listened to the pitch drop, ending with a ping when the reed snapped.
I phoned Colin Dipper, posted the broken reed to him and received a replacement by return of post. Colin said to ring him when I had installed it to discuss final tuning, but it was perfect so I didn't need to do anything.
This reed sounded a bit different for a while, but I can't even remember which one it was now.
Steve