Sequence is different..
GRACUSER Table is update by sync job , not by sequence.
Sequence is used to identify user.
Like your example: GRC backend system . if you search user first it will look for users in backend system , then it will go to other system maintained in sequence.
FF ID are not pulled may be your config is not proper.
check parameter for FFID role Name assign same role to FFIDS.
and in SPRO>GRC>Access Control > EAM you can define the role name for connector in GRC 10.1. that should also be the same.
Regards,
Prasant