Grafdigger's Cage effect partially persists after being destroyed

Stormcrow

Just fought a match in The Great Tournament where my opponent played Grafdigger's Cage. It was functioning correctly while in play. I destroyed the Grafdigger's Cage (by matching its gem) and after it was destroyed, my destroyed/discarded cards resumed going into my graveyard, as expected; however my opponent's cards continued to go straight into exile, as though the Cage was still in play, for the rest of the game.

This bug caused the "Millstone" optional objective to be completely impossible to obtain in that match.