> I’m sure I decline whatever cookie notifications they throw at me (except when some dark patten tricks me),
Okay I retested with a EU VPN and I can confirm it's due to you declining the cookies. If you decline cookies and change currencies, it adds your desired currency to the url as a query parameter, so if you navigate to flights.google.com again you lose it.
>but in any case, that sort of behavior shouldn’t be governed by any privacy-concerning cookies.
In other words:
you: "don't store cookies and data about me"
google: "ok, we won't store cookies and data about you"
you: "why didn't you store my currency?"
Actually if you read the cookie prompt that you declined, it specifically mentions that they use cookies for "personalization" purposes, and that if you decline they will not use cookies for that purpose. Storing your desired currency arguably counts as "personalization" purposes, so I don't really see the issue here.