I see your point. The help message is conflicting with the marker on that field.
Clients should always be set with a valid redirect uri (if they rely on grant types that involve redirects). If that means explicitly setting it or if calculated based on some other field, it does not matter as long as it is defined.
Not sure about UX best practices here, but my guess is that the marker exists to highlight what I explained above. If that is an "anti-pattern", it should be fine to remove the marker if we can make sure we always set one based on the root URL, when not explicitly provided.