Mapping with existing user not working in oAuth Custom authentication handler after upgrade

After upgrading from AEM 6.3 to AEM 6.5, my custom oAuth authentication is not working. The issue I found is oAUth handler is not able to map requested user with the user already present in CRX and returns "user not found" and authentication fails. Users I have migrated from AEM 6.3 to AEM 6.5.

Also in AEM 6.3, my custom oAuth provider was using the default "sync handler" and "external login module" but after the upgrade, I can see separate sync handler and external login module created for my OAuth provider but I didn't write any.

AEM 6.5 configurations -