After the first sort* of one item every following sort* of that item in the player inventory doubles it. It's the same using either the button or the hotkey.

e.g.
1st sort: 1 apple moved = 1 apple
2nd sort: 1 apple moved = 2 apples
3rd sort : 2 apples moved = 4 apples
...

This doesn't happen if the player sorts the inventory while having a chest open.
Closing and opening the inventory resets it back to the first sort(not in item quantity but in sort behaviour).

*that moves the item

Forge: 14.22.1.2480
Quark: r1.3-109

I can reproduce this, but only in creative.

Sorry, you're right, I was in creative and completely forgot about it.