I have an issue on v5.73.4 and could reproduce on CiviCRM Drupal Sandbox site.
I create a custom field set containing two select list fields (Committees and function in committee). Assign the custom field set to a contact subtype (eg: staff) the edit a staff contact. Adding the first entry works but adding additional entry to the contact looks as it works (shows the contact saved dialog) but the following entered values are never saved. So multiple values do not work anymore for custom field sets.
Another issue I noted is when you go back to the custom field set settings and remove the contact subtype (staff) to allow for any, you get the warning message saying you may loose data but the change is never saved. It keeps the contact subtype.