I don't see how this would be a bug, this is in keeping with what they've done previously in shard events. Last time the cards from the Shards 1 event were available, it seems like for the time being they're giving players 2 chances to get these trading post cards....Unless you mean him keeping the shards in which case that doesn't seem odd either, since in Pero Colo which Shards seems to have replaced, we collected "Pero Gacha Tickets" and those also carried over to the next event.