This gets even weirder.

Gunicorn behaves same as wsgiref.

However, it turns out they both only show the unexpected result if using curl. If you use safari they are both fine.

Waitress blows up altogether on it with an exception when you use curl as client, but is okay with Safari and gives what I expect.

My mod_wsgi package gives what I expect whether you use curl or Safari. So Apache may be doing some magic in there to allow it to always work. No idea. But obviously mod_wsgi rules as it works regardless. :-)

uWSGI doesn't want to compile on MacOS X for me at the moment.

That Apache works properly whether use curl or Safari and other WSGI servers don't suggests something is amiss.