Melendor in costume will not ascend

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
etc…

Read more here in the costumes FAQ thread:

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