This will teach me to read the beta release notes more carefully:
"Xcode automatically generates classes or class extensions for the entities and properties in a Core
Data data model. Automatic code generation is enabled and disabled on an entity by entity basis,
and is enabled for all entities in new models using the Xcode 8 file format. This feature is available for
any data model that has been upgraded to the Xcode 8 format. You specify whether Xcode generates
Swift or Objective-C code for a data model using the data model’s file inspector.
When automatic code generation is enabled for an entity, Xcode creates either a class or class
extension for the entity as specified in the entity's inspector: the specified class name is used and the
sources are placed in the project’s Derived Data. For both Swift and Objective-C, these classes are
directly usable from the project’s code. For Objective-C, an additional header file is created for all
generated entities in your model: The file name conforms to the naming convention
'DataModelName+CoreDataModel.h'."
Working as intended. Don't manually generate NSManagedObjectModel subclasses in new data models created in Xcode 8.