If you compile a subclass of StateMachineBehaviour into a DLL, the plugin importer will not be able to find the referenced assemblies and you cannot use the subclass in your project.

Seems isolated to 2018.2, does not occur on 2018.1 or 2018.3

1. Install Unity
2. Open the attached project
3. Open "New Animator Controller" in the Project View
4. Select an empty state
5. Select Add Behaviour in the Inspector

Result:
The class "CustomAnimatorState" is missing from the list of Behaviours.

Expected Result:
The class "CustomAnimatorState" should appear in the list.