Device Tagging Registry Key

I have been experimenting with using the https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-atp/machine-tags-windows-defender-advanced-threat-protection for populating machine tags but I have had trouble getting it to work as desired. Ideally we would like to organize the machines in groups based on their OU path. If I use the OU path in the registry key the tag never shows up in the console. If I use simple text the tag shows up. What are the requirements for the value of the registry key? Is there another way to group machines based on their OU path?  

Examples that work: "RegistryTagForDefenderATP12345", "Registry0Test0Zeros","Reg/Test/ATP"

Examples that do not work: "Workstations\Labs\Department\Information Technology", "Workstations/Labs/Department/Information Technology", "Workstations@labs@Department@Information Technology"