The _UtilityRegistrations object makes a reference to the Component's _utilities and _utility_registrations when it is created. The Component object holds a strong reference to the _UtilityRegistrations. Presumably the _UtilityRegistrations holds these two references to avoid holding a reference to the Component instance and thus introducing a cycle.

The problem introduced in 4.4.1 is that if either of those two instance variables are assigned to, the cache object is not updated. Previously, accessing the cache would compare the identity of the objects it held with the identity of the objects the Component held and reset if they changed. Now that doesn't happen.

The use case is that subclasses may want to change the identity of _utility_registrations in the registerUtility method. Specifically, we have a subclass that will change the _utility_registrations object to a BTree instead of a dict when it grows to a certain size.

Now we could update our subclass to have a dependency on >= 4.4.1 and knowledge of the fact that the cache is now an instance attribute that also needs to be updated, but I'd prefer to see it handled here so that the cache is always consistent and guaranteed to be so.