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How is that possible to pay a lightning invoice with

[nutshell][1]

and get Invoice paid Mint did not provide a preimage. as described in this issue: https://github.com/cashubtc/nutshell/issues/682

I though that lightning payments work like this: Upon the completion of the payment the recipient reveals the preimage to claim the funds. How would you explain this behaviour of the wallet / mint ?

[1]: https://github.com/cashubtc/nutshell