That's a new feature in Windows Server 2012. Not exactly a Windows Legacy OS.
Since the file backed up needs to be in a consistent state, the user is not allowed to change it during the backup. That makes sense.
https://community.spiceworks.com/topic/427919-user-profile-disk-backup makes me think there's more to it.
I suggest calling your Microsoft account manager for support? They must have foreseen this problem and possibly designed a solution.
I need to backup UPD (User Profile Disk ). When I use robocopy, it locks vhdx file and user logs in with temporary profile.
Does robocopy has some flag that avoid locking files? Maybe somebody knows another tool for this purpose.