Credit Reduction for Extension of IR Lock

Regarding a lender credit reduction for an extension of an interest rate lock, may a lender reduce a lender credit to extend the lock period? Example; A rate lock expired 4-25-19 and the closing date was 4-28-19. The initial rate lock and Loan Estimate reflect a lender credit of $2000.00 with 4.25% interest rate. The lock was extended through 4-29-19 at the same rate, but the lender credit was reduced to 1800.00. Since the lender credit was reduced based on an interest rate dependent charge, is this acceptable for interest rate extensions? If it is, is there a requirement that documentation confirms the delay was due to the borrower or outside of the lenders control?