Safari forces desktop switch while open
I am having the exact same issue. Safari kept stealing focus no matter which fullscreen app I would move to. It only does this when moving to a fullscreen app. I found a workaround that keeps it from switching completely away from the fullscreen apps. 

I unchecked "When switching to an application, switch to a Space with open windows for the application" in Mission Control. And this stops it from moving back to wherever Safari is open across spaces or screens (running with an external monitor). 

This doesn't fix the problem completely as Safari is still technically stealing focus as apps will basically twitch and the Touchbar will switch to Safari displaying the "New Window" "New Private Window" buttons. When switching to Google Chrome, the top part of the app will drop slightly as if the menu bar was being selected and then you have to click into Chrome again to get the focus back to chrome.