Co-Managed Devices reverting UseAUServer

Assuming you mean the UseWUServer registry value, setting this value in no way prevents a device from using Windows Update for Business. What's more likely happening here is that something is disabling dual scan on these devices or disabling automatic updates completely by setting the NoAutoUpdate value to 1. This could be a group policy, a configuration baseline from ConfigMgr, a startup script, or any number of other possibilities.