When you level up with two heroes the same, you normally get a 25% bonus to the special progression leveling.
When you actually level a costumed hero with an identical hero wearing the same costume, the bonus is not locked at 25% anymore, but 6% and that’s all you get. evidently I was obviously using 3-star heroes hence the 6% levelling bonus
At first I thought it was because my hero didn’t have the costume on and so I equipped it to check the bonus yet again and it remained at 6%
Because you are using a hero, not a costume. Sounds like you have simply equipped a hero with the costume and then fed it. That’s not the same as feeding a costume.