Your profile is set to private, so we can’t see your activities to try to determine what may have happened. My guess, though, is that you didn’t go quite far enough to reach the ‘finish’ banner/arch for the route.
Looking at your activity it looks like you stopped just short on Tempus Fugit. Your activity is 19.6km and the route with lead-in is 19.7km. It ends after you go under the stone arch (end of the sprint) for the 3rd time.
Your December 12 Out and Back again activity against stopped short. It ends at the stone arch (same as Tempus Fugit) and is 42.2 km long with lead-in