Not exactly the problem what the OP describes, I've had something similar all the time (for the past few years), but only for the US address, all other data saves fine. Eventually it will start working, might be a few days before the flight. What's annoying is that MMB shows 'all info entered', but when you exit and re-enter the booking, it shows 'provide more details' again.

I'm guessing this is related to the fact that as a US passport holder this wasn't required previously - but the USA Covid entry requirements now do enforce this.