I accepted a challenge invitation but the app froze when it started. (I had seen it when it was one hour old and accepted after two hours so it had not expired) So I closed and reopened the app. After that the challenge invitation had vanished without any trace.
What kind of device you have, and is this a periodic problem?
Salam. No, it happened only once. I got an elderly I phone. Maybe it was just a strange glitch.