Hi,
Perhaps it is caused by the default settings of Samba. You can check it out with the following command:
samba-tool domain passwordsettings show
hi
Password complexity: on
Store plaintext passwords: off
Password history length: 10
Minimum password length: 8
Minimum password age (days): 0
Maximum password age (days): 60
Account lockout duration (mins): 30 // i DONNO why but i can change this parameter
Account lockout threshold (attempts): 6
Reset account lockout after (mins): 15372286728