Unfortunately, I don't know how to reliably reproduce this yet so I can only hope what I have found so far is of some use.
Unity 2017.4.4f1
Wwise 2018.1.3.6784
Wwise 2018.1.4.6807 - still an issue
A Wwise Type in Unity will often lose it's path shortly after first setting it. For example, I created some states for Pause and None in the script. Wrote the code to set the states in the script. Flipped back to the Unity editor and set the publics to "Game/Pause" and "Game/None". Saved the scene, switched to the main scene, then ran the game. The states didn't work. When I came back to the Unity editor I found the publics reset to "Pause" and "None". I reset them AGAIN to "Game/Pause" and "Game/None" and then the states worked fine. There is no possible way for me to accidentally reset these things without their path value so it must be an internal bug.
This is not the first time this as happened either. The first time was with setting two switches instead of two states. Pretty much the same exact procedure. The weird thing there, however, was that only one of the two states lost its path. Being new to Wwise and finding little in the docs on Wwise Types, I lost a few hours on this when it first happened before I found why the states weren't working.
But, this doesn't happen all the time either. And, once reset, they haven't changed since. It seems to be only when first setting the types and possibly with changing scenes and jumping in to test quickly. I haven't had any problems with the types set on prefabs, for example. So far, just with types set on non-prefabbed scene objects with scenes other than the startup scene (I haven't tried to set any on the startup scene yet because I haven't gotten to menu SFX yet).
Update: Never mind that last theory, the path was just lost on one of the switch types in a prefab that's been around a while. There were other edits on the prefab and/or the script but no changes to Wwise Types on it. Basically, what works one day... somewhat randomly may not work the next.