You are correct. This is a fundamental problem with all *MPPS pools. When the balance is negative, miners know that their payment has a good chance to be delayed. It doesn't matter if the pool pays recent first (RSMPPS), strives for equal payment for all shares (ESMPPS) or pays all unsaturated shares every round (SMPPS). And, since this should quickly cause the collapse of the pool, "delayed" really means "gone forever".