Well, under several different circumstances, the slot acts like nothing is in it. Some people have said that it happens when you have Primed Mods in that slot, and some say that it's because you're using a certain amount of the capacity. I don't know what the reason is for certain, I just know that it doesn't let me use up too much capacity. I made a post in the Chimera Bugs Megathread, you can probably find it by viewing my profile and then recent activity. I made a more detailed post there. 🙂 Hope this helps!