Hello @Richard872,
Did you send a reservation change request for the extra 10 days?
If so the new reservation is then for 40 days so it's a long term reservation i.e. one of 28 nights or more.
Airbnb automatically applies the long term reservation cancellation policy to those no matter what the host's normal policy is. If you cancel such a stay the policy states that you pay for the 30 days after cancellation or up to the original checkout date whichever is sooner. In this case you'd have to pay for the extra 10 days if you cancelled on the 30th day.
If you booked the extra 10 days as a separate booking then the host's normal cancellation policy applies.
Steve.