Using stonecutter with Shift

I am playing Minecraft BE v1.11.1.

I tried to make smooth stone slab with stonecutter.

I wanted to make 2 stacks of them, so I  prepared 1 stack of smooth stone.

I put the smooth stone to the frame, and selection of slab was appeared.

I clicked that with a shift key.

I expected I could made 2 stacks.

However, I could only make 1 stack and all the smooth stone was lost.

That means I lost 32 smooth stone.

That did not happen without using a shift key.

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I am a Japanese, so there may me some grammatical mistakes.