I got a push notification about the issue, but didn't read it. On paying for a coffee this afternoon either my payment worked or the barrista didn't read the decline message - a little lesson for both of us to read and not assume :)
Also from the status page:
If you have incurred FX fees because you were unable to use your Monzo card, we will reimburse you for those fees. Please get in touch once this issue has been resolved.