@triddick, it just occurred to me what the difference likely is between your Clydesdale MasterCard debit card and the Monzo ones: does your Clydesdale Bank card use the LINK network (should have a LINK logo on the back)? I imagine it does, as I think all the main UK banks have their debit cards attached to the LINK network. Monzo, on the other hand, does not use the LINK network, and withdrawals only work with ATMs which are also connected to MasterCard’s network (which is the vast majority).
I imagine that if an ATM sees a LINK card, it assumes it is a domestic UK one (since the LINK network is UK-specific). It doesn’t matter whether your bank card is also a MasterCard or Visa debit card, from the ATM’s perspective, it is a LINK cash card. With the Monzo card, however, the cash is being requested via MasterCard’s network. Until now in the UK, this has only happened when someone used a MasterCard credit card, or foreign MasterCard debit card, to withdraw cash from an ATM. Hence the “your bank may charge you” notice, since almost all credit cards charge a cash advance fee for withdrawing from an ATM. And also the language choice, since the assumption is you are not from the UK, because you would just use your (LINK) cash card if you were.