if you run apache2+php, apache2 takes care of buffering between the two (if any). this seems to be working.
if you run nginx+php, nginx takes care of buffering between the two via fastcgi_buffers. this seems to be working (at least after you adjusted those)
if you run nginx+apache2+php, nginx takes care of buffering between nginx and apache2 via proxy_buffers. this seems to be to small for whatever your page/app does, hence you need to adjust those. (not fastcgi_buffers, because nginx does not talk to php directly in this case)