Unit Tests failing when I Run All Tests but pass when I Debug

The failing tests share a resource that affects them all when tested together. Recheck the affected tests and their subjects.

You should also look into static fields or properties in the subjects. They tends to cause issues if not used properly when designing your classes.

Some subtle differences might occur. For instance if a first test change a state which affects the behavior of a second test, then the outcome of this 2nd test may not be the same if I run it alone.

An idea to help understand a test failure when a breakpoint can't be used, could be to add logging.

Anyway, to answer your questions:

This ever happen to anyone else?

Yes

Am I doing something stupid or could there be a bug in VS2017 or NUnit or something?

I bet that it's neither: just a case a bit more subtle