Skype for business's share my screen shows only skype screen and desktop background
We have to explicitly add permission to share the screen in System preferences then SFB shows everything in the screen.

Ideally SFB should prompt to get some kind of permission to do this or let the user know about this through some message or notification so that it'll be obvious for user.Catlaina is released to public as well so I hope Microsoft address this issue soon

Please make the below manual choices to get rid of this problem


System Preferences 
Security and Privacy 
Screen Recording 
Check the Skype for Business option.