If usePopup is giving you problems, a workaround is to bypass the Apple JS SDK and open the pop-up manually. When I did this, clicking on "Continue" after the user verified their credentials redirected the pop-up to my redirect uri, regardless of whether or not the popup and redirect uri were on different subdomains.
Here's the sample code that gets called whenever a user clicks on my "Sign In With Apple" button:
const handleClick = () => {
				const oauthUrl = `https://appleid.apple.com/auth/authorize?response_type=code&` +
				`client_id=${CLIENT_ID}&scope=name+email&response_mode=form_post&` +
				`state=${STATE}&redirect_uri=${REDIRECT_URI}&nonce=${secureRandomString(32)}`
				const windowWidth = 450
				const windowHeight = 600
				const left = window.screen.width / 2 - windowWidth / 2
				const top = window.screen.height / 2 - windowHeight / 2
				window.open(oauthUrl, 'Apple Sign-In',`menubar=no,location=no,scrollbars=no,status=` + `no,width=${windowWidth},height=${windowHeight},top=${top},left=${left}`)	
		}
kachi
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If usePopup is giving you problems, a workaround is to bypass the Apple JS SDK and open the pop-up manually. When I did this, clicking on "Continue" after the user verified their credentials redirected the pop-up to my redirect uri, regardless of whether or not the popup and redirect uri were on different subdomains.
Here's the sample code that gets called whenever a user clicks on my "Sign In With Apple" button:
const handleClick = () => {
				const oauthUrl = `https://appleid.apple.com/auth/authorize?response_type=code&` +
				`client_id=${CLIENT_ID}&scope=name+email&response_mode=form_post&` +
				`state=${STATE}&redirect_uri=${REDIRECT_URI}&nonce=${secureRandomString(32)}`
				const windowWidth = 450
				const windowHeight = 600
				const left = window.screen.width / 2 - windowWidth / 2
				const top = window.screen.height / 2 - windowHeight / 2
				window.open(oauthUrl, 'Apple Sign-In',`menubar=no,location=no,scrollbars=no,status=` + `no,width=${windowWidth},height=${windowHeight},top=${top},left=${left}`)	
		}
Something is definitely up right now. Running into the same exact issue. Builds aren't appearing in Testflight or under the Activity tab after successful upload through XCode