This has a simple solution, everyone.
The FAA has listed "affirmative" in its documentation. That's fine. ICAO uses "affirm" as its standard.
As the ICAO is international, that doesn't mean that a country could have some other underlying standard or rule they want their controllers and pilots to follow. In this case, the FAA does have something underlying from what ICAO prescribes.
Work with me on this.
Say you live in a country which has a law that states that you must throw away your straw wrapper immediately after you open it. But there is also an international law that states that you must wait until your drink is finished before throwing away the straw wrapper.
Which is going to take precedence? Laws relative to your locale, of course. That doesn't mean it trumps international law, but that international law is supplemental to local law. Such is the same with "affirm" vs. "Affirmative". Both are right, depending on where you are and which regulations apply.
So as a pilot, if you fly from Gatwick to Vegas (VIR43, VIR44), you should probably be up to speed with both regulations in both countries, as well as ICAO regs, as ALL would apply; but local ones to that region would take precedence.