sqlalchemy - get_history doesn't pick up the change of the foreign key when instance is assigned, only when modifying foreign key directly

I have a before_flush event listener which checks if the employee's manager changed. In such case a record is automatically created in EmpManHist table. manager is self-reference to the Employee table. Here's my table definition:

class Employee(Base):
    __tablename__ = 'employees'

    emp_id = Column(String, primary_key=True, unique=True)
    name = Column(String, nullable=False)
    manager_id = Column(String, ForeignKey('employees.emp_id'))
    direct_reports = relationship('Employee', backref=backref('manager', remote_side=[emp_id]))

When I assign new manager to an employee by modifying the ForeignKey (manager_id) directly it is correctly picked up by the event listener. But when I do an instance assignment it doesn't:

# this code does not pick up the manager_id change in the 'before_flush' event listener
emp2.manager = emp3
dal.session.add(emp2)
dal.session.commit()

# this does
emp2.manager_id = '1'
dal.session.add(emp2)
dal.session.commit()

Why is that? I'm using python 3.6.3 and sqlalchemy 1.1.13

Below is the full working example:

from sqlalchemy import create_engine, Column, Integer, String, DateTime, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref
from sqlalchemy import event
from sqlalchemy.orm.attributes import get_history
import datetime


Base = declarative_base()


class DataAccessLayer(object):

    def __init__(self):
        self.conn_string = conn_string
        self.engine = None
        self.session = None
        self.Session = None
        self.echo = True

    def connect(self):
        self.engine = create_engine(self.conn_string, echo=self.echo)
        Base.metadata.create_all(self.engine)
        self.Session = sessionmaker(bind=self.engine)


class Employee(Base):
    __tablename__ = 'employees'

    emp_id = Column(String, primary_key=True, unique=True)
    name = Column(String, nullable=False)
    manager_id = Column(String, ForeignKey('employees.emp_id'))
    direct_reports = relationship('Employee', backref=backref('manager', remote_side=[emp_id]))


class EmpManHist(Base):
    __tablename__ = 'emp_man_history'

    id = Column(Integer, primary_key=True)
    emp_id = Column(String, ForeignKey('employees.emp_id'))
    man_id_from = Column(String, ForeignKey('employees.emp_id'))
    man_id_to = Column(String, ForeignKey('employees.emp_id'))
    when = Column(DateTime, default=datetime.datetime.now)

    manager_from = relationship('Employee', foreign_keys=[man_id_from])
    manager_to = relationship('Employee', foreign_keys=[man_id_to])


conn_string = 'sqlite:///:memory:'
dal = DataAccessLayer()
dal.echo = True
dal.connect()
dal.session = dal.Session()


@event.listens_for(dal.session, 'before_flush')
def _emp_history_update(session, flush_context, instances):
    print("BEFORE FLUSH")
    for instance in session.dirty:
        if not isinstance(instance, Employee):
            continue
        man_hist = get_history(instance, 'manager_id')
        if man_hist.added:
            if man_hist.deleted:
                man_deleted = str(man_hist.deleted[0])
            else:
                man_deleted = None
            emp_man_hist = EmpManHist(emp_id=instance.emp_id, man_id_from=man_deleted,
                                      man_id_to=str(man_hist.added[0]))
            session.add(emp_man_hist)


emp1 = Employee(emp_id='1', name="AAA")
emp2 = Employee(emp_id='2', name="BBB", manager_id='1')
emp3 = Employee(emp_id='3', name="CCC", manager_id='1')


dal.session.add(emp3)
dal.session.flush()
dal.session.add(emp1)
dal.session.add(emp2)

dal.session.commit()

# this code does not pick up the manager_id change in the 'before_flush' event listener
emp2.manager = emp3
dal.session.add(emp2)
dal.session.add(emp3)
dal.session.commit()

# this does
emp2.manager_id = '1'
dal.session.add(emp2)
dal.session.commit()

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