Figured it out. See Apple doesn't want the actual year and month from the Gregorian calendar, but rather the year and month of their own fiscal accounting calendar. So 01 is October, 02 is November, and so on. https://developer.apple.com/forums/thread/114240?login=true https://developer.apple.com/forums/thread/665639

Did you find a solution?

I'm also not seeing it for the past few months, since period ending 11/26. did you receive anything? https://api.appstoreconnect.apple.com/v1/financeReports?filter[regionCode]=ZZ&filter[reportType]=FINANCIAL&filter[vendorNumber]=123&filter[reportDate]=2022-12

According to this answer, it is the latter (store credit). https://apple.stackexchange.com/questions/453265/app-store-refund-when-a-user-upgrades-a-subscription-on-app-store?noredirect=1#comment658401_453265

For in-app purchases, I believe it's from here? https://developer.apple.com/documentation/appstoreconnectapi/app_store/in-app_purchase/in-app_purchase_price_schedules

also related: https://developer.apple.com/forums/thread/711031