Did you know there is actual ZPM Lamps? I dont know if they are still sold but that is what I saw during “Research” (aka looking for a ZPM in a Picture so I can model it properly)
I think there was a Bug once that it always filled the Inventory First before Hotbar, instead of only filling your Inventory when there is already a pre-existing Item that would stack with it.
So to make it short, it SHOULD fill your Hotbar Slot IF you dont already have an Item that would stack with the one you are grabbing. This is because some people want to grab the Item directly, like if it is a Thaumometer (those go into Shelves too, yes!) or an Extruder Shape and usually they dont already have a stacking Item in their Inventory when they do that. 