I too hope this gets fixed soon, but until then I would like to share another possible workaround.
The following line currently does NOT work (note 200):
/foo/* /.netlify/functions/app?foo=:splat 200
However this did the trick for me (note 200 and full url):
/foo/* https://example.netlify.app/.netlify/functions/app?foo=:splat 200
EDIT: Note if you use netlify dev with this setup, it will also route you to the production function. (not so useful after all)