Screen bug on windows 10 on iMac
Solution 1:
this seems like a software issue. Once you are booted into macOS, try reinstalling Windows support software from Apple via BootCamp. Once it is downloaded, install it to your Windows partition. Check this out for installation help: https://support.apple.com/en-us/HT204923
The problem with doing a manual install Is that it doesn't always install necessary drivers.
If the problem continues, then try doing a power on self-test. To do this, reboot the Mac and hold "D" while it is starting up.
Good luck!