So I was in creative mode to test something (water stream to move items) when I noticed my blocks disappearing. When you get a stack of a certain item eg cobblestone and you go into your inventory to throw in on the ground one by one (drag it outside of the inventory popup screen and rightclick) most of your items disappear. It seems like they are not actually thrown on the ground.  I've tested it a few times and it seems like 22/64 blocks actually land on the ground.

I hope this is clear. I've attached some screenshots of me testing this.