Symfony + PdoSessionHandler: Data too long for column 'sess_data' at row 1

Question

On my Symfony 4.4 application I use PdoSessionHandler to store session in the database. Yesterday I got an error log for one of the users:

SQLSTATE[22001]: String data, right truncated: 1406 Data too long for column 'sess_data' at row 1

I was surprised that sess_data could get so big that this error would appear. In the end it is a BLOB field. I checked other rows and noticed that CHAR_LENGTH could reach up to 50,415 and more. The session contents being 95% (hundreds of lines) of _csrf tokens.

Firstly, why sould session store so many _csrf tokens? What could I do to prevent it?

Here is a configuration for the session handler:

# framework.yaml
session:
  handler_id: 'Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler'
  gc_maxlifetime: 7890000  # 3 months
  cookie_lifetime: 31536000  # 1 year

# services.yaml
Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler:
  arguments:
    - !service { class: 'PDO', factory: ['@database_connection', 'getWrappedConnection'] }
    - { lock_mode: 1 }