Not receiving rewards in SweetTreat Showdown

same here! 

I think they only show in the inventory of whichever sim you were using when you opened the boxes and claimed the rewards, sadly :(

I have not gotten any of the rewards either.  Does not matter what sim it was.  

Yes, unfortunately this sometimes happens. I had the same problem last time, I had to repeat the event several times until I managed to complete everything and get the reward