This ticket can be closed/deleted. My test was flawed, and what looked like item deletion was actually the items being picked up by a nearby hopper that I thought was locked 😅

Putting any filled bucket – lava, water, powdered snow, fish, etc. – into a dispenser and triggering it in the latest snapshot (24w36a) simply deletes the item entirely. It doesn't dispense anything, doesn't drop the item entity – it's just gone forever.

Edit: Upon further testing, this happens with filled bottles, too; I suspect it's any item that acts like a container.