I am experiencing the same issues in Brazil. See these photos from last night. One says that the card does not belong to the network. The card is rejected by the terminal without attempting to authorise a charge, so it is a waste of time for @AndreasK to ask for exact dates or times of the transactions, as Revolut can see nothing on its systems. And please don’t suggest using contactless as a solution. Although most Brazilian card terminals bear a contactless symbol, most merchants don’t know how to enter an amount before the card is presented. The only procedure that they know on their card terminals is to insert the card first. Contactless is almost unheard of.
Before Revolut continues replacing UK residents’ MasterCards with Visa cards, Revolut needs to find a solution to this problem, otherwise Revolut is useless in Brazil for UK residents. Perhaps Revolut needs to offer a choice of MasterCard and Visa, so that all customers can choose the card type that matches their intended destinations.