I believe the problem is that type resolution function is unable to reach init due to its meta data. I replicated the problem locally, though I currently don’t know the source. Default methods are wired into Java in a rather odd way because they didn’t exist in the original specification, thus instead they appear more like methods in the derived class which in the case of Proxy doesn’t exist. It doesn’t seem to require them to consider the class complete as jpype isn’t producing a warning. Meaning if we can call the method, it would be fine.
Investigating further the problem is not actually JPype, but instead Java itself. There is no easy way to access default methods of a proxy because any attempt to call the default method simply redirects back to the proxy at least last time I checked. I managed to patch something in using MethodHandles. Hopefully that addresses the issue.