social-app-flask | Python Social Auth - Application - Flask | Authentication library

You should declare complex mixin columns such as yours using the declared_attr decorator. Simple column declarations will be copied from the mixin:

To accomplish this, the declarative extension creates a copy of each Column object encountered on a class that is detected as a mixin.

and apparently this does not play well with mutation tracking. The documentation is perhaps a bit vague on what constructs should use the decorator.

This copy mechanism is limited to simple columns that have no foreign keys, as a ForeignKey itself contains references to columns which can’t be properly recreated at this level. For columns that have foreign keys, as well as for the variety of mapper-level constructs that require destination-explicit context, the declared_attr decorator is provided so that patterns common to many classes can be defined as callables

Emphasis added, though I might've misunderstood what "destination-explicit context" means in this context. So do this: