stat.drop wont increase when Player throws Items out of his Inventory screen

I was messing around with a stat.drop scoreboard and I noticed that the stat wont increase when a Player is in an Inventory screen an throws some Items either with drag'n'drop or by hovering over an Item and pressing Q (drop).
Also, it wont give a player 20 points when he drops 20 Items at once via ctrl+Q (drop)

Fix appreciated

~ Halbzwilling

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Code analysis by Marcono1234 can be found in this comment.