Every instance I've seen for DCC so far in Portugal has been the double decline version. I went to Time Out Market, and I was looking for a merchant that had a customer facing terminal. (The food hall is cashless, so you either have to use a bank card or load a stored value card with cash.)

I was casually observing two customers in front of me, both with GBP denominated cards. The first customer got the DCC prompt (on the customer facing terminal) and correctly declined by pressing 2. At the second screen, he pressed 1 for accepting the conversion, and I could see that the receipt printed in GBP. I'm a bit surprised that the terminals aren't configured to switch the button order, such as, "Confirm decline conversion?" 1) Yes 2) No after having pressed 2 on the previous screen for local currency. However, at least in this one data point, it was enough to confuse the customer. The DCC offer in my case was $25.70 on a €22.25 purchase, so it was 4.70%, which is in line with what I've been seeing here.

The next customer was paying with a Visa branded debit card, and this card did not present a DCC prompt. Perhaps the transaction got processed via a debit network instead of through Visa?