I also never encountered this during all of my bookings over the timeframe mentioned (2018-2021) making me think that I had a selection of AUD in the Preferred Currency settings https://www.airbnb.com.au/account-settings/preferences
So I must assume that a default selection of USD or (no default) setting was the one affecting these customers. Strange that Airbnb, with their widely regarded bleeding user experience, still chose to render $ amounts without the default? "USD" qualifier.
Was this really deliberate? Seems likely given their attention to details like this.