Most of the companies where we encounter this behavior have no idea that they are still using FRS. In fact, after seeing symptoms such as this, I cautiously ask them the question, "So... do you think it's possible that you guys are still running FRS instead of DFSR?" and they always answer, "No way! We ditched FRS long ago!" But... so far I have a 100% success rate for being right; when we query Active Directory, we find that, sure enough, they are still using FRS.

You can only utilize one or the other to sync data between Domain Controllers. You have either cut your environment over to DFSR, or you are still running FRS. Don't automatically assume that just because you have Server 2016 Domain Controllers running in the environment that you are cut over to DFSR. Thankfully, there is a quick way to test and discover which replication technology is being used in your domain.

Log into a newer (2008+) Domain Controller server in your environment, and open up a Command Prompt. Then issue the dfsrmig /getmigrationstate command:

If it reports a Global state of Start, this means you are still running FRS. I say again: Start=Bad news!

If you see Eliminated listed, this is an indication that you are running the newer DFSR.

There are two additional possible outcomes to this command. If you see either Prepared or Redirected listed here, then it appears someone has started the migration to DFSR but never finished it.

If you discover that you are still running FRS, change it! Guidance on updating your SYSVOL replication scheme to DFSR is outside the scope of a Group Policy book, but here is a good starting point: https://www.microsoft.com/en-us/download/details.aspx?id=4843.