I'm currently in the process of setting up our apple-app-site-association file on our server and testing password autofill between our app and website (including the option to use an automatic strong password)
Everything is working as expected on iOS 12 (password autofill, strong password suggestions, saving password in the keychain, etc), but it's not working at all on iOS 13 (I started working on this in beta 7 and still trying in beta 8). On our "create account" screen, if I tap into the password field, I'm not getting the option to use a Strong Password. In the console I get:
Cannot show Automatic Strong Passwords for app bundleID...due to error: Cannot save passwords for this app. Make sure you have set up Associated Domains for your app and AutoFill Passwords is enabled in Settings
If I go to the login screen in the app, it's not offering the saved password that's in my keychain for our domain. Again, this all works fine on my iOS 12 device.
For the create account screen, we have the textContentType set to .username and the password field set to .newPassword. On the login screen, the password textContentType is just set to password.
The only thing I can possibly think at this point is we don't have the MIME type set to application/json for the apple-app-site-association file yet. Perhaps iOS 13 is more strict about that?