Recent studies show that for systems with four identical fermions in the j = 9/2 shell, two special states, which have seniority v = 4 and total spins I = 4 and 6, are eigenstates of any two-body interaction. These states have good seniority for an arbitrary interaction. In this work, an analytic proof is given to this peculiar occurrence of partial conservation of seniority, which is the consequence of the special property of certain coefficients of fractional parentage. Further calculations did not reveal its existence in systems with other n and/or I for shells with j <= 15/2.