Custom excerpt length filter doesn’t work

For anyone wondering, as custom excerpts don’t get trimmed by the excerpt_length filter hook, try adding this filter:

function trim_custom_excerpt($excerpt) {
    if (has_excerpt()) {
        $excerpt = wp_trim_words(get_the_excerpt(), apply_filters("excerpt_length", 55));
    }

    return $excerpt;
}

add_filter("the_excerpt", "trim_custom_excerpt", 999);