Sorry, I found a similar post and see it is a known bug
When running the game in full screen, and I alt+tab out, the icon disappears from the Taskbar and I can’t alt+tab back into it. I can still hear it running in the background and the processes are all still running in Task Manager.
If I run the game in Windowed mode it works fine and I can navigate away from it and back in no problem.
Geforce GTX 1060 6gb
Windows 10 build 1803
All drivers up to date
All Windows updates installed
Nothing funny in Event Viewer showing a crash/freeze
Verified Integrity of files already but have NOT tried a reinstall