This is easiest to describe exactly how it happened:

I had a full inventory, with a half-stack of cobble in my offhand.

I broke cobblestone, and the items stayed on the ground, as opposed to me picking them up and them being added to my offhand stack. (I did not have any other partial-stacks of cobble in my inventory elsewhere.)

I just think it would be helpful and would make gameplay... smoother... if it would just pick up the items into the stack.