I have a copy of melendor in costume that will not ascend even though he is at lvl40. Get a message that costumes ascension cannot be higher than heros ascension
Unfortunately not a bug.
By Design, the costume cannot be ascended beyond the point of the base hero.
I.e. if your base hero is a 4* hero & only on the first ascension, the costume cannot go more than 1-40
similarly, if it is in 2nd ascension (2nd chevron), the costume cannot go more than 2-50
3rd ascension on a 4* hero caps out the costume at 3-60
Read more here in the costumes FAQ thread: