Had this same issue, did the work around and it SEEMED to be working. I completed all but one of the tasks for both challenges - the only one left being the seance, I have paranormal and planned to do this when I could afford to to complete the week and get all the rewards. Instead, when I came back from entering cas two discovery quests popped up. Doing them or cancelling them made no difference, the reaper rewards one would not reappear.
It doesn't show as 'completed' on the main event menu like the other challenge does. I cannot get the final reward for the week, I cannot complete the challenge. I tried starting a new save without discovery quests to see if the reaper reward event would pop back up, even if just the last one again, and it did not. So I'm just sol I guess.
It's the sims 4 so I wasn't expecting polished and flawless implementation, but this does seem shoddy.
Edit to add: After this I moved the new account data file to a second safe location and was resigned to just move on with things, but forgot to move the original back in, so when starting up game again it generated a new one. When I then went back into my save I'd done the event on, it re-triggered both reaper reward challenges to start from scratch. All the rewards were still in inventory and unlocked in CAS and Build/Buy. Wasn't able to get dupes or anything, just finish unlocking what hadn't been and complete things fully.
Then did the final step for people with the older event items and was able to get those back and keep this stuff. All of that worked, and was able to get it all for use (Although I'll be curious to see if have to repeat the process of removing the file next week.) SO. If anyone else has the vanishing reaper rewards issue come up, try removing the accountdb file again??? It worked for me at least.