What is the second pickup/interact option for this grenade?
Solution 1:
The second option is the same for guns as it is for grenades. Holding the pickup button will instead equip the item and replace it with the one you currently have equipped.
The equipped one will be placed in your inventory. If your inventory is full you will drop the currently equipped item.
This is what it is supposed to look like. It should say 'Take' and 'Equip'.
I still believe this ~4.3s to be a bug.