Shift-Click Diamond For Anvil Clears All But 1 Diamond

What I expected to happen was...:
Based on what I was trying to do I figured it would add only the highest amount of diamond possible. (So if 2 diamond could be placed to repair my item, only 2 diamonds would be placed out of my 10).

What actually happened was...:
What actually occurred was all of the diamond went into the box, then all of the diamond except 1 got deleted. And it was not just a client bug, I relogged a few times and tested it with different amounts, all amounts did the same thing.

Steps to Reproduce:
1. Get a sword (have not reproduced with other items yet) that you wish to repair. In my case it was a Smite 3 diamond sword.
2. Place the sword in the first slot of the Anvil.
3. Have a stack of diamonds in your inventory and shift-click them so it will "place" them in the second slot of the Anvil.
4. Notice the diamonds disappear in this second slot and only 1 diamond remains (permanently).