As I've noted in several previous threads, I spoke to the staff at Barcelona and - while aware of the issue - at the time they didn't know what was causing it as there's seemingly no common factors among people experiencing it.
At the time there'd only been four weeks, and while the first week worked for everyone (it seems), some people were experiencing issues with the second and third sets (I was among them) but the fourth set was working fine.
For reference, I was also dicking about with the system clock to get the information sooner for the GTP guides those weeks, but other people also did that and had no problems.
I've also been deliberately trying to screw it up since then with other things that people have reported:
*Completing Daily Workout at the same time as a Weekly Challenge event
*Having a Menu Book open during Weekly Challenges
*Suspending a race overnight and coming back the next day
*Completing them in different orders
None have resulted in missed rewards.