SharePoint 2016 user profile service issue

We had migrated the content from SharePoint 2013. (did not migrate the UPS, only content). The old farm was still running and the UPS was running on the old farm. As soon as that was shut off, the AD Sync imported correctly.

Doesn't totally make sense, but I suspect something in AD was getting confused in regards to the change log. They were different servers and different accounts so no totally sure unless they were attempting connections at the same time maybe.

Not truly sure, but it is now fixed.

Looks like there is some profile corruption issue. as per this MSDN link. you have to follow these steps:

1. Run following powershell command and make note of guid of UPSA. Get-SPServiceApplication
2. Run following commands one by one. $u = Get-SPServiceApplication Set-SPProfileServiceApplication $u -GetNonImportedObjects $true # Here you will get the results for profiles 'IsImportedvalue = 0' from ProfileDB Ran Set-SPProfileServiceApplication $u -PurgeNonImportedObjects $true # This cmdlet will mark the profiles for deletion and need MySiteCleanup job to delete those which are marked for deletion. If MySite is part of one user that added in missing profile, the same job will delete those MySites as well. Here is the detailed implications while running My Site Cleanup job.
3. Run Incremental profile import. Query following against ProfileDB and we should get no results. select * from DNLookup where isimported !=1 # Make sure you have backup of profile database before running any queries against it.
4. Go ahead and run Full profile Import, once the above query returns no result.