You are very welcome and that additional context makes the situation much clearer.
Given your experience with 1Password, I would not send you through the usual basic troubleshooting again. The interesting issue here is specifically the iOS Mail account-password-change prompt, rather than 1Password's normal AutoFill behaviour.
Your description also explains why the usual workaround failed:
if that particular credential field exposes neither AutoFill nor Paste, then iOS is effectively giving you no way to transfer the generated password into it.
That is an Apple/Mail UI limitation rather than something you are overlooking in 1Password.
For the next occurrence, there are a couple of things I would want to investigate specifically:
- whether the prompt is a native iOS Mail authentication dialog or a web/authentication view presented by the mail provider;
- whether iOS's AutoFill & Passwords integration is actually offered in that particular credential field (your discovery of that setting is useful here);
- whether the account can be removed/re-added or its credentials updated through Mail's account settings instead of through the sync-error prompt; and
- importantly, whether there is a way to get the password into that prompt through the iOS share/keyboard/1Password mechanisms even when the usual Paste command is suppressed.
If this happens again, a screenshot of the exact prompt would be enormously useful (with your email address and any other identifying information obscured).
That should let us distinguish the different kinds of Mail authentication dialogs without requiring you to deliberately change your Comcast password just to reproduce it.
And yes—given that you have been using 1Password across Mac, Windows, iOS and Android for a decade, “just use Apple Passwords” is very much not the answer here. 😄