Uncertain whether or not this is intended (only Mojang can tell us that) but I'll put this up anyway.
When the "pick block" key (which I map to to left ctrl due to my laptop only having a two-button touchpad) is used in creative mode on a block that you have in your survival inventory (not the hotbar), it takes the block from your inventory and moves it into the hotbar; if the hotbar is full, it switches with the current hotbar slot.
However, when this is done, you end up with exactly one of whatever block you picked, regardless of how many items were in the stack originally.
Steps to reproduce:
1. Be in creative mode.
2. Put a stack of several of one kind of block (say, Grass Block) in your survival inventory (not the hotbar). Make sure there is only one stack of said block, and there is more than one item in said stack.
3. Find the block you chose in the world and use the pick block key on it.
4. The block will appear in your hotbar. Observe that the size of this stack is 1, as indicated by the absence of a number.
5. Open your survival inventory and observe that the stack you originally had is gone.
In creative mode, I put a stack of 64 stone blocks into my hotbar. I place some stone in the world, then move the stack to my survival inventory. I then use pick block on the stone I placed. One block of stone appears in my hotbar; the stack of 64 is nowhere to be found.
Again (I'm unsure) this may or may not be intended, and also I acknowledge the possibility that it could be a duplicate of
MC-13053 (resolved "Works as Intended"), as they both involve the "pick block from inventory" feature; however that issue deals with renamed blocks, while this one deals with the sizes of item stacks.