Expired domain name auctions are typically started before the domain name expires, during the redemption period, and so it is possible for an auction to end early if the current registrant renews the domain or even for a refund to be issued after the auction ends and a winner has paid. If you participate in GoDaddy auctions, you will encounter this with maybe 25% of all domains that go to auction. That said, my understanding of Namecheap's exclusive access to expired .ai domains is that they only auction .ai domains after they have been dropped (meaning they cannot be renewed).
Was this an expired domain auction, or was it a user-initiated marketplace auction?