What I expected to happen was...:
A single water bucket would empty when placing water and would still remain in the hotbar as an empty bucket waiting to be refilled again.

What actually happened was...:
The water bucket emptied, but also moved inside the inventory. This is problematic if you wish to place plenty of water without crafting additional buckets due to the scrolling hotbar present in Minecraft Pocket Edition.

Steps to Reproduce:
1. Open up your inventory.
2. Select the bucket.
3. Pick up water using an empty bucket.
4. Open up your inventory.
5. Select the water bucket.
6. Place the water at desired location.
7. Open up your inventory.
8. Select the empty bucket.
9. Collect water using the empty bucket.
10. Open up your inventory.
11. Select the water bucket.
12. Place the water at your desired location again.
13. Repeat steps 1-12 to continue placing more water

This would be more efficient if players would be allowed to place the items or blocks they wish to use in the hotbar. While keeping them there without having them disappear by scrolling off the screen after selecting another block or item.