How can a line manager assign a journey?

Summary:

We have a requirement for a line manager to be able to assign one particular journey to their direct reports. We have enabled the ORA_PER_JOURNEY_SECURITY_ENABLED profile option, and we have added the privilege "Manage Journey by Manager" to our custom line manager role.

On this particular journey, we have configured security as seen here:

When going into the Explore tab to this journey, the line manager can click on this journey, click Assign in the top left of the screen, and then only assign it to themselves. No one else in the line manager's organization is available in the dropdown for assigning.