Do you have anything written by Apple concerning this?
We lost mobile home sync, now network accounts altogether. Next step I guess it's macOS Server
To be clear — network
accounts still work; but user home folders stored on a central server (known as portable home directories, mobile home sync, etc.) are no longer supported as of 10.12. Your users will need to transition to using home folders on the local Mac rather than the server. I would suspect this is the cause of the Internet Accounts issue you're seeing.
It sounds like in the past, your users had network accounts with documents, preferences, etc. stored in home directories on the server. When they logged into a workstation, it would download their home folder from the server. When they logged out, it would sync any changes back up to the server.
Now your users will still use their network accounts; however, their documents and preferences can only be stored on the Mac they're logging in to. Data will not transfer automatically across multiple workstations.
Starting with macOS Sierra, you won't be able to create portable home directories. Mobile home directories, which have networks accounts that are cached locally, can still be created. However, their home folder will no longer sync with their network home directory.
from:
https://support.apple.com/en-us/HT206871
