This is not a bug, the Extreme items have limited option on the way you can add slot types.
If you first add for example 1 slot of CDI then for the second one you will add ASA you will be able to add on the 3rd one only CDI, anything else will fail but you will have no slot removed.
The extreme items can have either 2 slots with the same value and 1 different or 3 slots with the same value.
This is how it works. We will think of an way to make this more obvious for the players ingame or make an detailed tutorial on the cabalwiki.