Judging from the last thread about Sixt here, because they probably got a warning from Mastercard or something, they accept the cards when you order online. But later they send an email to the customer about them actually not wanting to accept the card. It's not in line with the Mastercard policies, which is why they're probably trying to hide it like this.
You see officially they don't admit to it: https://www.sixt.nl/help-center/articles/betaalmethoden/#/ Here they only say that prepaid cards are not accepted (which is a valid exception to make, but bunq cards aren't prepaid).
But then they send these message to customers. Unfortunately it doesn't seem like Mastercard is taking enough action against them.
