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

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.

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.

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.