When you discard items from the inventory by dragging the mouse or using "CTRL + Q" slowly, the number of items remains the same. But if you use "CTRL + Q" and move the mouse over the items in the inventory to eject them very quickly - the number will change (decrease). More in the videos below:

Steps to reproduce:

1. Take at least 18 stacks of any item)

2. A - Only discard items by taking them to the cursor and discarding them from the inventory. This is without error - the same number. 

    B - Discard items using "CTRL + Q", but slowly (interval +- one second) - the same number

    C - Discard items using "CTRL + Q", but very quickly. Hold "CTRL + Q" and just move the mouse over the items - different number of items. (This one is broken)

3. Pick up the items to see the difference

Observed Results:

Items disappear and the player loses his progress.

Expected Results:

Items will not disappear after using a quick "CTRL + Q"

Note:

I saw this bug in a previously reported bug, but it was closed as incomplete. (MC-88852)

Thanks, krampi