This seems to be related to an apache mod_proxy_uwsgi bug.
For those stuck in same situation, I used this workaround :
- configure uwsgi proxying on a TCP socket (ie uWSGI on a TCP socket)
- check apache2/mod_proxy_uwsgi.c and apply following fix if needed : https://github.com/unbit/uwsgi/commit/6ce856c9fdb98833f9142f84c92b75f546b32dc2#diff-85cd090aa23e09feb1f28c92ae5aded6