Using Elegance theme, with category icons on, category icons are only rendered for categories with categoryID up to 20.

To recreate,

• choose or create a Moodle instance with at least 21 course categories (or at least 1 course categoryID > 20)
• turn category icons on in Elegance /admin/settings.php?section=theme_elegance_categoryicon
• Search categories /course/index.php or /course/index.php?categoryID=<parent categoryID which includes a child category with categoryID > 20)
• See that only categories with ID <= 20 have the category icon rendered. The others use the same CSS structure to render content, eg below:
  .course_category_tree .category[data-categoryid="21"] > .info > .categoryname a:before { content: "[[setting:categoryicon21]]"; }
• Note also in /admin/settings.php?section=theme_elegance_categoryicon that the maximum possible setting for "Number of categories" (theme_elegance | categorynumber) is 20.

Consider the use of a default category icon where the setting is unavailable, or increase the maximum number of categories.