Buckets, when named and used, will lose the name given.

When you name a bucket, whether it be an empty one, or a lava/water/milk-filled one, it will be named, but upon using the bucket (filling or emptying), it will lose the name that you originally gave it. Even if you bring it back to the bucket-type it was originally named as, it will revert to its item identity, rather than what you named it.