NotListeningException after successful StartHost()

I’ve got a big suite of tests, which worked fine before I added NetworkObject components to the prefab under test.
Now, instantiating such a prefab requires a running NetworkManager.
In the SetUp of my test, I run NetworkManager.StartHost() which nicely returns true, all the time.
In the UnityTearDown, I run NetworkManager.Shutdown() + yield return new WaitUntil(() => _networkClientStopped) to be very sure that it has been shut down.

Running each of my tests individually, they pass.
Running a suite of tests, the first passes, but all the following tests fail with a NotListeningException upon spawning the prefab under test.

How can the NetworkManager not be listening when StartHost() finished successfully?