Just tried with keepInventory disabled, as in the video. I can confirm that if you are having the item in cursor (i.e. you are in the process of manually moving the item), the item does not despawn.
At that moment, the item is not in the inventory, nor is it in the world (as an entity). It is in temporary state.
[Not from a dev… Just to add precision]
One thing to note, if you have keepInventory game rule set to true, then I think that this is work as intended.
The video in previous comment clearly has keepInventory enabled.
Just a dumb question, but I see a command block. Could it be configured to setblock a block of bedrock continuously?
Just to rule that out…!