PasswordPolicyControl.graceAuthNsRemaining falsely returns None

[droideck]

While testing 389-ds-base on RHEL 7.5, we've encountered the issue with PasswordPolicyControl.
It doesn't return the remaining number of grace logins (graceAuthNsRemaining).
Openldap util ldapsearch returns the information on the same setup.

Steps to reproduce:

1. Install 389-ds-base on 389 port and 'dc=example,dc=com' suffix with 'cn=Directory Manager' and 'Secret123' password
2. Run the test case
   https://fedorapeople.org/~spichugi/ppolicy_grace_test.py

389-ds-base:
389-ds-base-1.3.7.5-18.el7.x86_64

Operating system:
Red Hat Enterprise Linux Server release 7.5 (Maipo)

Python: 3.6.3
python-ldap (3.0.0)

Additional information:
The original test case can be found here:
https://pagure.io/389-ds-base/blob/master/f/dirsrvtests/tests/suites/password/pwdPolicy_controls_test.py (test_pwd_expired_grace_limit)