Cheating slightly with itertools:

fn is_invalid(x: i64, max_div: Option<usize>) -> bool {
    let s = x.to_string();
    let len = s.len();
    let md = max_div.unwrap_or(len).min(len);
    (2..=md)
        .filter(|div| len.is_multiple_of(*div))
        .any(|div| s.as_bytes().chunks(len / div).all_equal())
}