Personally I don’t care if it is Monzo’s problem or not. My account is with them and I expect them to apologize for any interuption to service, however caused, and whoever caused it.
If they started rambling about Acquirers or Merchants or electronic point of sale terminal providers or any banky waffle it would look like they were washing their hands of it, despite it impacting card holders. While I may grasp the techie terminology the average customer probably won’t. It would be seen negitively and leave a bitter taste in the mouth.