Talking with Brett; the fix should be as simple as keeping a reference to the debug function which we have in the imports. During interpreter shutdown, the sys.modules is iterated and each module replaced with None. Since the _handle_workers thread persists slightly past the point of the parent (and can, it's a daemon thread) debug is vanishing on us.

We can go with switching this to a classmethod, and keeping a reference on the class, passing debug directly into the _handle_workers thread (testing this last night fixed it 100% of the time)