The bug

If you drop multiple stacks using ctrl+q some of them might disappear. Example: grab 27 stacks and hold ctrl+q and move your mouse over your inventory you will only get 14-19 stacks back when you pick them up again, they simply disappear.  

Have tested this both in single-player and multi-player (vanilla server) and I always come up short, not depending on gamemode, the stacks just disappear on me.

This also happens if you have several droppers spitting out items.

The game simply can't keep up with the items thus deletes them it seems.