Getting a stack of TNT from a dispenser

What I was expecting to see was....:

I was getting a stack of TNT from a dispenser what I expected to see was the 64 TNT in my inventory

What actually happened was...:
Instead what I saw was the 64 TNT still in the dispenser and the green bar floating over it

Steps on how to reproduce:
1. Put a stack of TNT into a dispenser
2. Long press the TNT until the green bar is full
3. The green bar will float over the stack of TNT and the stack will remain in the dispenser