Recreate:

Create a PlayerBucketEmptyEvent listener.  Set event.Cancelled(true).

What happens:

Bucket empties as normal and reverts to Material.BUCKET.

Open an inventory block (chest, furnace, crafting table etc.), bucket reverts to what it was before the event (Material.SALMON_BUCKET in my case).

I'm trying to access the ItemMeta on the original bucket and only cancel the event if the Meta matches a given criteria.  I can't get the ItemMeta until it reverts when I open an inventory block.