Does a KTR get the entire FFP monthly invoice even if a protest delays the contract award and the first month is reduced from 30 days to 15 days?

We have a contractor that asked us if they should prorate the first monthly invoice to match the actual days they worked/performed due to a delay in the start date because of a protest. The contract type is FFP, and my first thought is since this is a FFP pricing arrangement, they are entitled to the entire FFP monthly amount, but I honestly don't know if I am missing something or not thinking through it entirely. Would love any input or experience in dealing with a similar situation.