I believe the problem may be that we can't readily tell the difference between "PYTHONIOENCODING=ascii" and "PYTHONIOENCODING=ascii:strict", and in the former case we'd ideally still end up using "surrogateescape" by default.

That said, the real intent of the change was "If the detected encoding is ASCII, enable surrogateescape automatically", and detecting the POSIX locale was a proxy for that. We didn't account for PYTHONIOENCODING being used to select a more sensible encoding.