Virtual Desktop Agent (VDA) machines can be configured to use either WCF (default option) or Web socket protocol when communicating with the delivery controller. If the delivery controller is configured for using WCF protocol, the VDA should also be configured for WCF protocol.

Caution! Refer to the Disclaimer at the end of this article before using the Registry Editor.

Disclaimer:

"Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it."

Virtual Desktop Agent was setup to use Websocket protocol whereas the Delivery Controller was using the default WCF protocol.

Re-configuring the VDA to use WCF communication by setting the following registry keys will resolve the issue.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CitrixBrokerAgent\WebSocket
 
Name: Enabled
Type: REG_DWORD
Value: 0

One of the Windows 2022 VDA machines fails to register with Delivery Controller and remains in unregistered state. From the CDF traces, it was noticed that the affected VDA was using Websocket communication whereas the working VDAs were using WCF Communication and the DDC was also using WCF communication. 

https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/secure/websocket-vda-support