I have a similar problem on my desktop PC.
My configuration is:
Main display: 1080p 165Hz
Secondary: 1080p 60Hz
Windows and graphics drivers are all up to date.
Already tried DDU, reinstalled drivers.
Tested all hardware and cables working properly.
When secondary monitor is connected, main display turns on and off repeatedly.
I tried lowering my main display to 144Hz, 120Hz, 100Hz, 85Hz. and the problem only disappeared when set to 60Hz,
which is the same refresh rate as the secondary display.
Disconnecting the secondary display also eliminates the problem and allows me to set the refresh rate at 165Hz.
Obviously, this is not a solution.
I hope Microsoft or Nvidia fixes this soon.