Oh, it was nostr:npub1k3g092rlzvn7nftz3jte9pkx63zp705nh78r6hjpjm55fjg7r2cqx8stj3's built-in wallet, but in my own experimental build. I might have screwed something up. I'm not entirely sure what the project uses for cashu.

Looks like I screwed up my wallet. Talked to the coordinator and now I have a valid preimage of the transaction. Wrote you a DM.