When you pick up lava, water, or powdered snow with a single bucket the item does not convert into a bucket and instead disappears. Same thing happens when you place an entity Bucket (axolotl, pufferfish, salmon, tadpole, cod, tropical fish), Lava Bucket, and Water bucket.

What I expected to happen was...:
getting the bucket item back after using the full bucket / getting the full bucket when using the bucket

What actually happened was...:
the bucket disappeared 

Steps to Reproduce (1):
1. be in survival mode
2. make a bucket
3. pick up lava, water, or powdered snow
4. the bucket disappears

Steps to Reproduce (2):
1. be in survival mode
2. place a water bucket, lava bucket, or entity bucket
3. the bucket disappears