This doesn't belong in the Developers Area, and should instead be posted within the Graphics / Drivers Section of the Forums.
With this said, if your Game (Fullscreen Exclusive) and Desktop are running at different resolutions... or your have Virtual Super Resolution (VSR) enabled, then it will take a few moments for the Card to Sync with the Display and Switch to the correct Resolution.
If you know that you're going to be "Alt-Tab" between Game and Desktop, it's recommended to switch to Fullscreen Borderless Mode as the Resolution is retained allowing for Instant Switching.
thanks alot it worked !!