I updated to 2.12 a little while ago. First thing I did was open the demos solution, in VS 2019 16.6 preview 5. After fixing my local Nuget pointer to 2.12, I did a Nuget restore, then did a solution Build. 100s of warnings in the output window. Don't recall this when building previous version Demos project.
Then created a new project using same VS version, and chose Telerik CRUD/Form/Chart template, Blazor server. That build is throwing the Async warnings I'm familiar with from the WeatherForecastService Error CS1998 about the lack of an awaits keyword. Is this something you mean to suppress in Editor.config?
Same warnings thrown during build with VS 2019 16.5.4