Not sure if this has been covered yet.....I searched so forgive me if it has -
I don't think this is bug more of a design oversight -
You have 5 special outfits to make as your own , then you have a costume section.
Problem: when you have a costume selected it overwrites and takes the slot of the current selected outfit - this then reduces your amount of special outfit slots and at the same time doubles the entry for that particular costume.
I think this needs to be corrected so that a costume selection is it's own selection and has no impact on the special outfit slots otherwise technically you have to scarfice one of your special outfit slots in order to be able to use your outfit slots and costumes when changing appearance on the fly.
The special costume replaces the skin you have in that slot but it will go back to the created skin when you take the special costume off so in reality you have more than 6 skins available. The 6 ones you create and have available in slots 0 to 5 plus the special costumes (I have 2 so thats 8 for me).