Repro:1. Create .Net solution with 2 projects. One from built-in .Net Core -> Class Library and one from .Net Core -> Unit test project templates.
2. Change TargetFramework to TargetFrameworks and net462 in both projects.
3. Write some tests.
4. Test runner will see tests, but will not run any of them.
Tests will also fail to run if You set TargetFramework unstead of TargetFrameworks and add following package reference to main lib
<PackageReference Include="System.Threading.Tasks.Extensions" Version="4.4.0" />
This type of projects became more popular now, when people are preparing to multi-target existing projects to both full framework and .net core