It seems pretty clear how this should be handled, no? If the app wants to do it, then, as with anything else that might make the user experience better at a privacy cost, let it ask for permission to do it!
Having to ask for the permission to display the email just for the autocomplete makes sense from a privacy perspective, but defeats its UX purpose.
A better solution would probably be to continue to move away from email + password logins and ask the user to login once in an OCD platform and then only propose this in order to signup/login to an app.