An enWiki search for deepcat:"Musicals by topic" returns 87 results. But a visual inspection of the category tree shows that it contains well over 100 pages. For a more specific example, a search for deepcat:"Musicals by topic" intitle:"Kid" returns 1 result, (Kid Boots), but Kid Victory is at least one other example of a page that should have been returned. Both pages are in Category:LGBT-related musicals which is a direct subcat of "Musicals by topic".
This category should not be hitting the documented limits of max depth 5 and 256 max categories. It contains only 8 direct subcats, and 1 grandchild category (https://en.wikipedia.org/wiki/Category:LGBT-related_musical_films), nothing else. This category is in fact the one used as an example on Wikipedia's search help page.