When a custom field is set as required on resolve, and the issue is updated from an unresolved status directly into a closed status, the custom field is not validated.
For example, with standard resolution thresholds: updating the status from "assigned" to "closed" does not validate such custom field.

When talking about resolution thresholds:
Since "closed status" threshold should be a higher value than "resolved status" threshold, a closed status is effectively also a resolved status, and the requeriment for this custom field should also apply.