The reason for this is, you can collect it again once the cooldown period is done with. At least... this is how it is with the old Collection Folders.
Yea, I know you can collect them again, but I would love to see the green tick turn back into the grey icon after discarding it.
Don't drop collection folders at the NPC instead do it through the bottom menu.
I likely will do it that way from now on, but it still makes sense that if you can discard them in the collections NPC, and she already uses the green tick one screen out, that they could add the tick to the item inside the menu until it is discarded
EDIT: I also kinda like being able to discard the previous collections and accept the new ones in the same screen. Not sure if the bottom menu has that, but I doubt it.