When I pay with Monzo, recently Deliveroo will take the money from my account and then instantly refund me and say the transaction failed. Usually I have to approve the transaction via the Monzo app but the authorisation is never triggered and it just fails. I have more than enough money in the account as well.
And yes, all other cards work fine (mastercard and visa)
I’ve never had to approve anything or had any issues with using Deliveroo with my Monzo card, used it most recently yesterday. Best to contact Monzo.
To do the above 
Tap into one of the transactions, scroll to the bottom and tap 'Something wrong? Get help"
have you tried to use apple pay or Gpay within the app instead to pay with your monzo card? that’s what i’ve been using and it works fine