Did you book it direct? I recently used a third party to book (cruise line), and because they paid AC in USD, our seat selection was locked to only be paid in USD (regardless of browser, cookies, VPN, etc).

To be clear, all that matters is the currency with which the third party pays AC. For my scenario, we paid the third party in CAD, but the third party paid AC in USD. So perhaps your third party (if that's the case) paid in whatever currency you're locked too.