By default, the email verification flow breaks the OAuth/PKCE context because the user leaves the registration page to check their email and clicks a verification link. FusionAuth can’t continue the OAuth flow automatically from that email link, which is why your hard-coded redirect URI fails without the required OAuth code.

A better solution is to switch your email verification strategy from Clickable Link to Form Field (under Tenant → Email → Email Verification → Verification Strategy). With this approach, users stay on the original registration page, enter the verification code from their email, and the OAuth/PKCE flow remains intact—including the authorization code. This enables seamless redirecting back into your app after verification.

If you’re creating and registering users for an application at the same time, remember to also check the app-level registration verification settings under:
FusionAuth Admin UI → Applications → Edit Application → Registration tab.

More details are in the docs: Registration Email Verification