What about instead of

'''
Whether changes to one are reflected in the other after the call returns is undefined; additionally, the dictionary may change unpredictably after the call, and how it does is implementation-specific.
'''

substitue this wording:

'''
Whether changes to one are reflected in the other after the call returns, and when such updates occur, is undefined and implementation-specific.
'''

The old wording seems under-specified. It would allow a function call, garbage collection, etc, to clobber the dictionary, say overwriting with another function’s locals(), before you get a chance to work with the dictionary.