Not sure if this is a known bug, I know there are some issues with XC/Arriva on board purchases but this was online. Current Account card was just declined online due to a “system error” - not the usual “declined because you don’t have enough money”.
Yup and on Twitter but it’s out of hours so not expecting a response until tomorrow
Edit: CrossCountry are escualting it (along with another bug report I made!) and I’ve asked that they email Monzo to figure something out. Hopefully should be fixed soon