*I’m not good at English,I’m apologize, but i’ll tried to communicate your guy.
*And not guarantee will be fix for all of issue.But it work for me.
At First,
I change sound system from sound onboard to usb headset (Logitech G633).
And back to start the game it back to desktop again.
Now I
1.Go to Windows All Setting
2.Go to App&Features
3.Find Forza Horizon 4 Demo
4.Click the Advanced options
5.At App permission make sure Microphone is slice to Off
6.Click The Reset
And Now Fun With Forza Horizon 4 Demo
It run At 3413x1440 with 21:9 monitor very smoothly.
My System
i7 6700k
Main board Gigabyte (Forgot the series)
Ram 16Gb 3200nhz
Galax GTX1080ti
sound on board (for watch movie,music)
Logitech G633 (for play games)