ShakeOff re-allocates puzzle pieces that have already been allocated

I have been playing ShakeOff 2020 for a while now, and have been excitedly collecting puzzle pieces in order to try to win some of the cool prizes. Recently, I have been getting messages that the puzzle piece has already been allocated.

Is this the way that the promotion works?

So, in a 6-piece puzzle, the app only allocates a subset of pieces, and you never get to a position where you actually get all the pieces? I don't think that this fair, also extremely disingenuous of VC to waste users' time like this.