Sharing a desktop from Skype for Business using a Microsoft Teams Room system

Working in hybrid mode, when inviting someone to join a meeting from the Microsoft Teams Room touch panel, they are invited to a Skype call.  That's fine.  

But … the invited user cannot share any content with the Teams Room system, it times out.

If a second person is invited, then sharing works fine.

Anyone else seen this, or know why it happens?

This has been tested with different hardware solutions, so is not an issue of the platform but the MTR solution.