Earlier quoted context omitted.
This. There's so much funny business that goes on with FX rates that it makes me almost pro-crypto. Even the people who claim to offer raw spot market rates (ie. Interactive Brokers), in fact do not offer that. I would not be surprised in Visa & Mastercard do the same--although they claim otherwise.
Come to think of it, would Visa/MC be doing the conversion? I believe that's the issuing bank's job. But they offer the rewards, not Visa/MC, so it doesn't change the original answer much.
A lot of banks do charge a percentage fee on top of Visa/MC's exchange rate, but my bank does not.