Hi @Donald368
Thanks for the heads up as many new Hosts may not be aware of this. If you have flexible or moderate cancellation policy and a guest checks in but doesn't complete their stay, you will only receive a partial payout. Some guests will only book properties with a flexible ble or moderate cancellation policy, if they are not sure they will be completing the entire stay.
It's good you didn't accept the reservation alteration as well, as that restarts the cancellation window and the guest can simply cancel within 48 hours of your accepting it and get a full refund. Some guests are aware of this loophole and will try to get around your cancellation policy by shortening the stay via an alteration request (if they haven't checked in yet).
I believe the only way to avoid this is to go to a Firm (or more restrictive) cancellation policy, which does affect your guest search rank. If you want to extend a special offer in the future for a longer stay, you could turn off Instant Book, change your cancellation policy, then send the special offer. To be fair, you should advise the guest of the stricter cancellation policy for the special offer. After they accept the offer, you can turn instant book back on and revert to your moderate cancellation policy.
Cancellation Policies
https://www.airbnb.com/help/article/475