Associating your domain will allow keychain/autofill, but does not seem to allow SAVING credentials. In other words, if your credentials are already saved from visiting the domain in safari, then presented with a wkwebview with an app with trusted domain associated, the credentials will fill. BUT, BUT.... one cannot save credentials from this domain associated wkwebview because the save dialogue is never triggered. If anyone has any insights...

Still no updates to date. What is frustrating is that there is no acknowledgement or documentation from apple regarding wkwebview supported features that cover this.

This is the million dollar question. Autofill works, meaning the domain (and entitlement) is honored, but the save dialogue is never triggered.