Version 44.0.0, Build 1634
I just noticed this today, though it may have existed in previous versions, but the display of heroes from the Level Up button on the hero page (without selecting an actual hero) appears to have a couple issues. It looks like similar issues were reported in the past (where a maxed level but not max skilled hero was having issues - but these reports were from 2019). The issues I found are slightly different.
When you first select the Heroes button, all of your heroes are displayed and selectable. Note that the Non-Costume Grimm is maxed, and Zila has only been taken through the 3rd ascension (though she requires 4 ascension levels to be maxed)
But, when you select the Level Up button without selecting a hero first, it appears the selection does not take Ascension Level nor Costume Level into account. Note that both Grimm and Zila are disabled. The issue with Grimm will be shown on the next screen shot. Zila is fairly obvious - even though she may be maxed at her current level, she is not a fully maxed hero.
As for Grimm, while his base card is maxed, I do not have his costume maxed…
In both cases, I would have to select the card itself, in order to feed/level the hero.
As far as costumes go, is it possible to add a note (similar to the way Class Quest Hero Selection works) that says “Costume may allow levelling” If the costume is not fully leveled.
Similarly, if a hero is maxed at their current level, but is not at their max ascension level, can they be made available from the general “Level up” option.