In general, since the C accelerator is enabled by default, and few people would consider disabling it explicitly, I generally consider the behaviour of the C implementation to be "right", if both implementations differ.

As a single data point, the reason why the difference was found in this case was differing behaviour in PyPy (which uses only the Python implementation). It was only later found to be a problem on the CPython side.

Changing the behaviour of the C implementation would certainly break a lot more code than changing the Python implementation.