It turns out that this bug reproduces with any subclass of the generic type with a weakref slot, even without any multiple inheritance going on.

For example:

class Invertible2(Invertible[KT1, KT2]): pass

is enough to trigger this bug along with the Invertible class in my previous example. Attaching the more minimal repro with this comment, and renaming the issue to remove the reference to multiple inheritance.