I have Visual Studio 2019 16.6.2 installed, along with the .NET Core 3.1.301 SDK and all the necessary Blazor templates.
If I create a brand new Blazor WebAssmbly app
It appears that the issue is that if you remove the IIS Express section from the launchSettings.json file of your project, that debugging will then fail to work. And this is true even if you're using VS code against the project directly without IIS proxying or VS.net too.
So the solution is to put back the IIS Express section in the launchsettings.json file and not remove it until this is fixed.