I think the bureau has clarified the definition. From page 379 of the rule:
The Bureau has clarified the meaning of delinquency in commentary in a manner
substantially similar to the late payment trigger that was proposed in § 1024.39(b). Accordingly,
in the final rule, § 1024.39(a) requires a servicer to provide the written notice not later than the
45th day of “a borrower’s delinquency.” Comment 39(b)(1)-1 contains a cross-reference to
comment 39(a)-1, which generally explains that delinquency begins on the day a payment
sufficient to cover, principal, interest, and, if applicable, escrow for a given billing cycle is due
and unpaid, even if the borrower is afforded a period of time after the due date has passed to pay
before the servicer assesses a late fee. The cross-reference also clarifies that a borrower is not
delinquent for purposes of § 1024.39 if the borrower is performing as agreed under a loss
mitigation option designed to bring the borrower current on a previously missed payment.