Contact editing in accounts we sync is problematic in the default Android Contacts Editor.
It can be solved by using a different Contacts Editor. It’s an issue with the Android contact book – it can’t edit & create contacts synced accounts by some of the 3rd party applications.
You can install “Contact Editor Free” and use it as your Contacts Editor instead of the Default Android Application.
It works just fine and doesn't require registering having an account in their service. Select it as the application used to create & edit contacts.