If in creative mode inventory and item is picted it dissapear (it will not show in players inventory).
Best way to test this is to set cursor to look at sky because when item is droped in this position it will fall to the ground and it will be picked up. Go to brewing screen and press Q (or any key for droping items) on any potion 15 times and wait until all items are picked up. Now if you go to survival inventory items will not show up.
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Duplicate of MC-87019
Please check if that's still an issue for you in the latest 1.15.2 stable release or the latest 1.16 development snapshot.