Yes, the broken areas of the test class are all async methods. The dummy test I added stops working, if I turn it into an async method returning a task and adding a Task.Delay(100) to the method body. I tried for 2 hours to create a repro, but I am unable to reproduce the phenomen in the repro project. The strange thing is, that in my test projects it is also only affecting a single class. Other test classes within the test project (even async test methods) work fine. I am not sure, how to come up with a repro for this strange issue. I can't share all the code with you, but we could either setup a remote session, so you can have a look or I could maybe share the VS generated PDB and the NCrunch generated PDB. Not sure, what would actually help you.
Maybe there is also another way. E.g. could I force NCrunch to use the VS generated PDB (to verify its working with that one) or somehow make the VS and NCrunch PDB comparable for you?