The module hierarchy file includes all the classes that are implemented in the DEPENDS dependency chain. The wrappers, however, also require information about certain classes that are implemented outside of this dependency chain. Specifically, if a class header contains a forward reference to another class, and if that other class is used in the interface, then the wrappers require a hierarchy file that provides information about that class.
A solution to this problem is for the wrappers to use the hierarchy files for all modules in PRIVATE_DEPENDS, in addition to using the module's own hierarchy file. This will bring in more hierarchy files than is strictly necessary, of course, but too many is better than too few.