Cannot complete the Bandits Quest in Act 2

Returning player, came back after a while. One of the characters has the bandit leader quest active (which I'm certain I have completed before 3.0 update). Active quest stage is to gather the amulet shards for Oak. However, other bandit leaders, when killed, do not drop amulet shards.

it dosen't really have it active, you have a reward already, it's just showing as active because the rewards have changed, you can talk to leo in your hideout (he may offer you a book to change which bandit you pick if any) or you can do it by using the vendor recipie.