The answer was posted to my question on Stackoverflow: https://stackoverflow.com/a/65211685

Same here! When trying to checkout it brings up an Iframe to my credit card issuer, then as soon as I interact with the window in any way, it navigates away and denies my transaction. This is happening with two different cards from two different (U.S.) banks. My apps have all be de-listed because of this! You'd think Apple would do better QA than we all have to; where's their "Approval Process"?