Cannot Assign Multiple Roles to Case
I believe the issue, certainly as per the youtube video, is because the Case Contact is not of type Individual, while the Relationship you are trying to use, Benefits Specialist, is only for Individual to Individual. This unfortunately results in a silent failure. So if there is any 'fix' required here, it would be to provide an error msg saying "this Relationships is not available due to mismatching Contact Types".

We hit a similar 'why can't we reassign a case to a new case manager' and the issue in our case was that Case Manager relationship was set to be Individual to Individual but in this case the Case Client was an organisation.

The reason we were able to create the Case Manager in the first place was because these were added via webform which seemed to create the Relationship despite the Contact Type not being appropriate.