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

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.