Multi device user app in Flask App + SQL Alchemy - flask-sqlalchemy

Question

I developed a Flask App that uses SQL ALchemy as DB Layer. As it needs user registration and login, added Flask Security Too (https://flask-security-too.readthedocs.io/en/stable/).
The problem that I am facing is that when a user starts multidevice sessions (i.e. logs in on a site, an updates data in other) there is a conflict at SQLAlchemy ORM that finds that the same instance of a model is shared among different data base sessions.
SqlAlchemy: Insert New Row and Modify Another: "Can't attach instance <ObjectT>; another instance with key is already present in this session"
Regarding that, I have studied about merging objects, transition objects, and neither of those seems to be a valid solutions.
Without going into the code, what I am trying to figure out is "which is the strategy that a multi user multi device session needs to have to use SQL Alchemy?"
are using scoped sessions?
is SQL ALchemy a good fit?
Is there any other alternative to Flask Security Too that avoids facing this kind of problems?
More Data:
My database instantiation:
ONOS_SQLALCHEMY_DATABASE_URI= os.environ.get('BONOS_SQLALCHEMY_DATABASE_URI')
engine = create_engine(BONOS_SQLALCHEMY_DATABASE_URI, pool_size=14)
SessionFactory = scoped_session(sessionmaker(bind=engine))
session = SessionFactory(expire_on_commit=True)
Base = declarative_base()
The code that generates problems when I try to save changes on a session:
class SQLAlchemyDatastore(Datastore):
def commit(self):
self.db.session.commit()
def put(self, model):
self.db.session.merge(model)
self.db.session.commit()
self.db.session.add(model)
return model