Just the other day when i updated my graphics drivers, I began having issues with Stretched resolution and alt tabbing on fortnite. Whenever i play on my typical stretched resolution of 1656 x 1080, my screen goes black for roughly 2-3 seconds before coming back and switching the tab. I did monkey with some of the compatability settings in fortnite to disable fullscreen optimizations however I had this before hand and I never had the issue.
System Specs:
Ryzen 7 3700x
RX 5600 xt sapphire pulse
Asus TUF gaming x570 plus wifi
32 gb DDR4 3200Mhz ram
512gb inland premium nvme ssd
2td Seagate barracuda hard drive
850 w psu i believe