Hi Robert Putten,
I'm not certain, but the issue might be related to the hairpin effect. The release notes describe how the optimization used to eliminate the hairpin effect leverages the thin-client to be used for a media path. The notes specifically state that the, "media path does not go through the hosted virtual desktops (HVDs)", while it is in VDI optimization.
"To reduce latency and to enhance media quality, the VDI plugin optimizes the media workflows by streaming media directly between users on thin client endpoints and leverages the hardware of the thin client machines to handle media processing. This media path does not go through the hosted virtual desktops (HVDs). The result is a stable and full-featured calling and meeting experience for your VDI users."
Link for release notes: https://help.webex.com/en-US/article/ntp1us7/Webex-App-%7C-VDI-release-notes#Cisco_Concept.dita_99e48166-6e5c-446f-9d14-2dd8da7015c1
My guess is that you will not be able to share the HVD while in VDI optimization. I hope this helped and I await to here from anyone else within the community pertaining to this inquiry.
Best,
Corbin D.