When the hotbar is filled, and an item is picked up with the creative menu open that can't stack with anything in the hotbar, then the item disappears. However, if the same item is picked up with the creative menu closed, then the item reappears in the inventory stacked with the newly picked up item.

Replication:

Step 1: Fill up the hotbar with items

Step 2: Open the creative menu (Excluding the survival inventory) and drop an item so it is picked up with the menu open (For example, one magenta wool)

Step 3: When moving to the survival inventory, the item is gone

Step 4: When picking up the same type of item with either the survival inventory open or the entire creative menu closed, then the item will reappear (For example, if another magenta wool is picked up, then two magenta wool will be in the inventory.)