I have the same situation.

No squiggles lines AND Error List window doesn't show any errors if "Build + IntelliSense" is selected. It does show errors if "Build Only" is selected but then I have to manually rebuild after every edit.

I did remove and reinstall IntelliCode from the Visual Studio Installer,
Tools-Options-..-InteliCode unchecked and checked back
Tools-Options ... show error squiggles unchecked and checked back
Deleted .vs folder, and reset all settings to C# profile.
.obj and .bin folders deleted multiple times

Is GitHub copilot required for this to work? I hope not...
Is Live Share required? I hope not either...

I seems to be out of options any additional suggestions would be appreciated.