If you reassemble the puzzle with the broken piece, it still works ok, you just need to be careful with it.
Mine has the same problem, but it is still useable.
Thank you for this confirmation. With much effort and several attempts I finally did get it back together yesterday. I'm not sure that it matters, but success came when I used the broken area as the place to insert the last middle piece.
Also with a bit of experimentation, I found an easier way to get the last corner in than the previous method used months ago. It involved trying to twist as many pieces out of the way as possible so there was a clearer path into the groove.
And finally, I was able to scramble and solve it a few times with no further incident, but, as you said, I was extremely careful and deliberate with each move.
Thanks to all for the help and encouragement.