I had a similar issue trying to book two tickets for HKG-HAN and DAD-HKG. First one couldn't pay with Mastercard, 2 different Visa, ended up working with a one-time Revolut Visa card. Tried Revolut Visa for the second trip directly after, and failed again.
Next day, Revolut wasn't working again, Mastercard fail again, 1x Visa failed and 1x Visa finally made it.
Nightmare.