This absolutely needs to be fixed and it's so ridiculous that Apple still hasn't done it, but I came across a decent workaround if anyone can't wait for Tim Apple to wave a magic pencil:
https://codepen.io/callumfindlay/pen/VaYeoW
iamskwerl
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Also encountering this issue. Anyone out there?
After a couple of days, my logins magically started working. There must be a delay between creation of sandbox accounts, and synching up with Apple Pay systems. Not sure what other explanation there could be; I was certain I was using the correct passwords, and they simply didn't work until they did.