Ran into a similar issue (on Ubuntu 18.04) recently when upgrading my monitor except my screen also went black when the game was fullscreened. I originally set the refresh rate from the GUI going to Settings > Displays > Refresh Rate > 144.0 hz. When looking into my nvidia settings it detected 144hz, but upon launching OW it only showed options up to 2560x1440 60hz. When I set the resolution and refresh rate manually with xrandr and relaunched OW the option to select 2560x1440 144hz appeared and the black screen in fullscreen mode went away.
Here’s the command I ran to set resolution and refresh rate manually xrandr --output USB-C-0 --mode 2560x1440 --rate 144.00. Run xrandr to know what to set the output flag for your set up.