Just a note; been travelling across Japan now for 3 weeks and cash is definitely king. Where possible I have attempted to use Monzo in MasterCard locations however it has been rejected around 50% of the time. The card is not accepted by the card machine and I have to incur charges on my AMEX.
I don’t know if Monzo can see these attempts on my card but it would be great if you can highlight the issue to MasterCard and see if they can validate reasons why.