This is not a _bug_ that we ever saw in testing. Usually, the only time a gong piece temporarily goes missing is when the villager carrying it gets interrupted along the way. If that happens, closing the game, launching it again and repeating the puzzle solution (if necessary) is all that's required to retrieve it. There was a thread here that dealt with an issue getting the mosaic gong piece that was never resolved.

Is it possible that time warping is involved in any of the games that are having this problem? It would help to know all of the circumstances that led to the problem.