Frederik You should report them at MasterCard. They are breaking the "Honor all cards" rule from Mastercard. And maybe also Article 10 of the Regulation 2015/751/EU from the EU, but I'm not sure.
A merchant can choose to only accept Mastercard prepaid cards, or only accept Mastercard debit cards for example. But if he accepts debit cards, he is not allowed to discriminate between different issuers, and should accept all Mastercard debit cards.
PS: bunq is written in lowercase. It's supposed to be symmetric, so that it's still bunq when written upside down.
