@alex-2 Hmmm, curious.
What is the redirect URI or redirect URIs of the application configuration?
Is there any additional debug information when you run this in the iOS simulator?
Where are you running FusionAuth? Is the SSL certificate self-signed?
Are you using a webview? That's not typically recommended, as using safari (ASWebAuthenticationSession, to be precise) is the better path. One possible solution would be to use AppAuth, an iOS library, and see if the issue is present there.