Hi Mate,
Unfortunately not a bug. This topic has been discussed a number of times, most notably on the Costume FAQs page, Levelling a Costume & Costume Common Mistakes (Links below).
In summary:
- Costumes are applied to each hero
- Each costume must be levelled individualy… Costume levels are NOT transferable.
- Costume levels can exceed the level of the base hero but CANNOT exceed the ascension level of the base hero
- You can equip/not equip the costume. but as above, the costume levels are NOT transferable. each must be levelled individually.
Links: