Good question, and I should know by the end of the month when my Clarity statement is generated.
I took out cash from a cash machine in Iceland and Curve registered it as an ATM withdrawal as expected. My underlying card was a Clarity, but as far as I can see, the Clarity has not registered it as a cash withdrawal. I won’t know for certain until my statement is generated though and I can check if any interest was accrued.
I know that doesn’t 100% answer your question with Monzo but if my Clarity can’t pick up on the fact it was a Curve cash withdrawal, then I doubt Monzo will either.
My tactic in Iceland was to use Curve with Clarity underlying set up in GBP for weekdays, and then change my Clarity to ISK in the Curve app for the weekend. It seems to have worked fine as Curve did pass on all transactions in ISK when I changed it, which is what I wanted to avoid weekend markup.
All in all we’re talking about pennies being saved in my case, and being slapped with a ~£6 ATM fee in Iceland (apparently this was unavoidable) does put things in perspective a bit. Between my Clarity and the Curve card, I reckon any savings I made switching between them are completely wiped out by that one £6 charge. Oh well!