Owner fields that are polymorphic are not visible in Custom Report Types

What Polymorphic fields are

Polymorphic fields are defined as fields that can reference more than one Object. For example, a Lead or Case Owner can be either a User or a Queue record since the Lead Owner and Case Owner are both Lookup (User, Queue) fields.

Why Is the Owner Field Hidden for Community Users?

If a polymorphic field is included in a Custom Report Type, and a User doesn't have access to all the Objects referenced by it (User and Queue in the example above), then the field isn't visible for that User.

Resolution:

A polymorphic field is one that can reference more than one type of record. For example, the Case Owner field can reference either a User record or a Queue record. If a user does not have access to all referenced objects (e.g., Community Users cannot access Queue objects), the field is not visible in Custom Report Types.

Note:
Queue Object access level can't be granted to Community Users via standard sharing process, such as custom profile or permission sets. Hence, when a Custom Report Type is based on an object that uses a Queue as the owner, the Owner field isn't visible from the report.