Bug report: Many Cached hierarchies often do not reflect the current CodelistVersion state

Summary

Many cached hierarchies do not reflect the current state of a CodelistVersion, leading to incorrect assertions regarding the CodelistVersion's CodingSystemRelease compatibility.

What did you do and what did you expect to happen?

Expectation:

>>> cv = CodelistVersion.objects.get_by_hash("3f51e9b7")
>>> cv.hierarchy == cv.calculate_hierarchy(cv.coding_system)
True

Counter-example:

>>> cv = CodelistVersion.objects.get_by_hash("5b45576f")
>>> cv.hierarchy == cv.calculate_hierarchy(cv.coding_system)
False

Impact of the bug

When loading a new CodingSystemRelease, a CodelistVersion's cached hierarchy is compared against one calculated using the incoming release. Whether or not they are found to be the same is considered as part of whether the CodelistVersion is "compatible" with the release.

If the cached hierarchy is an inaccurate representation of the current state of the CodelistVersion, then it may lead to incorrect assertions of incompatibility of the version with a release.

Reproduce the two CodelistVersion examples using the hashes in the issue and compare hierarchy with calculate_hierarchy(cv.coding_system). Trace how the cached hierarchy is stored or refreshed, then add coverage showing it matches the current CodelistVersion state and no longer produces incorrect compatibility results.