One Time Billing Rules should only appear against the month specified under Start Month within the Billing Rule, credits assigned in this manner can rollover to subsequent months if the credit isn't exhausted as per - Credit Rollover
If you notice that a One Time billing rule appears in subsequent months from the start month, and in the case of a credit was completely applied in the start month please contact CloudHealth Support.
CloudHealth Support can remove the credit via an internal tool so please submit a ticket if you believe this is required.