Typically feeding the same hero to itself at max will automatically give you 100% chance to lvl up a special skill. I am trying to feed Rigard to fully leveled Rigard costume (in costume) and it only offers 40%. Not sure what is going on. Prior costume levelling provided the 100%-chance-at-max bonus when the hero/costume was maxxed.
Not 100% sure, but I think you must feed another Rigard costume to a maxed Rigard costume to get 100% chance.
Feeding Rigard costume to Rigard costume gives 8% chance. So when maxed it will give 40% chance. So it is working as intended. The following thread has all the relevant info.