Postgresql role_table_grants grantee differences between users
One possible cause is that UserA belongs to "admin_group" and that UserB does not belong to "admin_group".
Please post output of following statements run by a superuser from psql CLI:

\du userA
\du userB
select * from information_schema.role_table_grants where grantee='user_group';
select * from information_schema.role_table_grants where grantee='admin_group';