How to refresh workflow approvers when a field is updated on form?

We are currently using Primavera 17.10. We are attempting to design a BP that will require the submitter to select the sub-site as part of filling out the form and are using a data picker for this purpose. The sub-site is then being used as a filter condition to assign users from a permissions group as approvers in the workflow. The problem we are running into is that the group populates in the To: field on the form on form load, but it does not refresh with the filtered users after the sub-site is chosen. Instead the user gets a