Cognito doesn't validate with external IdP during refresh token flow, if the refresh token that is issued by Cognito is still valid, end-user can continue to get new access and id tokens from Cognito without needing to re-authenticate with the external IdP.
If you have a use-case that requires validation with external IdP then I'd recommend using a short-lived refresh token (1 hour is the shortest TTL for refresh token) and this will force sign-in when token expires.
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