NUnit recognizes all tests, but does not run all

Question

I am using NUnit 3.0.1 and Visual Studio 2015 and I can confirm that my problem does not occur in Visual Studio 2013.

After cleaning and building the solution, Test Explorer shows that another 100 tests have not yet been run. But when I click "Run All" in the test explorer, only ~ 20% of single tests are launched, the rest remain grayed out and are still marked as not yet running.

However, if I clean and build again, it again shows 100 tests that are not already running, but if I right-click the "Do not run tests" group and select "Run selected tests", it will run all the tests correctly.

My tests are currently contained in [TestFixture] , and it seems that only 2 out of 10 of my [TestCaseSource] seem to be running.