Iron bucket vanishing

When I add lava buckets to to furnace with a hopper underneath, going to a chest. Occasionally I find the bucket disappearing. Like it has never been crafted. I started off with 6 buckets, ended up with 3 (not sure if it's to do with location or not but it happened to me in the Nether.

 What should happen is that I should be getting my empty buckets back and not having to craft more and more.