Copying the GitHub response here.
This has lots of self intersecting geometry which can have an undefined result. It is hard for the algorithm to find a solution. I tried to repair it with NetFad and it gave an error. I would recommend you try to repair it with another mesh tool (possibly meshmixer or meshlab. Or if you can re-model it.
Here is a screenshot of the fixed mesh.
As you can see, it could not detect the complete whole correctly.
We will keep this as a test model (if its ok) to try and improve the algorithm, but it seems like this may not be fixable.