Hi Reno

The problem could be the method used by the desktop capture application to 'scrape' the screen. There are a number of different methods to do this, including calls the windowing system and calls to openGL.

its possible that the app you are using is calling openGL and that is failing on the USB connected screen - since that is more rather unusual connection - is this a Thunderbolt 3 to HDMI or a traditional USB2/3 to HDMI ?

There are at least 4 different Desktop capture apps for Mac to NDI, so trying them all may give you some perspective. However, not all support multiple screens

Some that do

- Sienna NDI ScanConverter (full version, Lite version only supports main screen)
This works fine with a second screen connected via Thunderbolt 3. Demo version at: Sienna.tv/ndi / Purchase here
- Vmix Desktop Capture download
This works fine with a second screen connected via Thunderbolt 3
- NewTek NDI Scan Converter (free) request
This displays all screens as a single image rather than selecting one or the other, but again it displays content from a second screen connected via Thunderbolt 3

If you are using a truly USB connected HDMI, then you should do the same test - you may find that one or the other works. Start with the Sienna NDI ScanConverter (demo if you don't own it) - since I can confirm that this does NOT pull pixels directly from openGL and so may have a better chance of getting the USB connected pixels.