@Graham146
@Gordon0 highlighted the contradiction in your making a 3rd Party Booking.
It seems there is also another discrepancy which best explains the situation.
You state 'We received an instant booking ...'
However, your 2 listings don't display the Instant Book icon.
If they did, you would surely be entitled to automatically cancel without an automated review of Cancellation.
So, it rather seems you received a Reservation Request with the message informing you of a 3rd Party Booking. If that's the case, then it seems prior to Accepting you overlooked the message... and then belatedly read the message and decided to Cancel.
If this is correct, then Airbnb will assume it was your responsibility, in which case CS will not oblige you in this matter. Airbnb will only act on behalf of the Host if the booker failed to disclose a 3rd Party booking till after the booking.
In respect of the current situation, if correct, i.e. you were informed of a 3rd Party Booking with the Reservation Request, Airbnb will instruct you for future reference either to Decline such an RR or request Guest to Cancel.
In sum, if my appraisal is correct, the automated Review will stand and you're wasting your time contacting CS.
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ps. I don't do Instant Book. In the past couple of month I contacted CS twice re Declining 3rd Party Reservation Requests, notably after Guests ignored several communiques my explanation and request for them to Cancel the RR. (Some people are simply inconsiderate and can't be bothered and leave Host to sort it.)
To Airbnb CS, I expressed my concern that my Acceptance Rate will be affected because the computer will not be able to distinguish the reason.--I was assured it would not be the case, as accompanying request for explanation for Declining is monitored.
Whatever the policy and assurances by inexperienced CS, the reality is different. My Acceptance Rate was affected as I predicted it would.
Consequently, in any future case, I will ask Airbnb to contact Guest to inform why they should Cancel rather than leave Host to Decline.