Exotic Gear Question

I know you can have only 1 Exotic Gear fitted at a time.

I also have a couple of what might be called exotic attachments (they are also red) for my turret:

"Cyclone Mag Housing Slot" and "SHD CPU V.2 Firing Mechanism Slot".

When I already have a piece of Exotic gear in my loadout, i.e.: Acostas Backpack, it will not let me add either of these attachments to my Turret.

Is this SOP or am I missing something?