Now, more than a year after your post, it is still a problem! No fix in sight.
I just got off the phone with Airbnb support. The bot (programmed by Airbnb) is still messing with us.
HOSTS NEED TO BE ABLE TO ADD INFANTS TO A RESERVATION VIA A CHANGE RESERVATION REQUEST.
The Airbnb platform is not allowing us to do so. When we attempt to do so, it gets to the point of making the adjustment and has the button to send the request to the guest, but then it displays an error message "Infants aren't counted as guests, so you don't need to change this reservation", and will not send the change request to the guest.
What would it take for reservations to accurately reflect the number of adults, children, and infants we will be hosting?