Since the webform CiviCRM module to 6.2.2 the option to add a custom set related to the contact sub type when selecting the contact sub type under CiviCRM webform settings.
Drupal: 9.5.2 Webform civicrm: 6.2.2 CiviCRM: 5.57.1
To Replicate:
- Create a custom set for contact sub-type Student
- Create a webform and enable CiviCRM.
- Select contact sub type Student. No Custom set is loaded to add on the form.