Hey there,
We put this question to the team who works on our proxy and they explained that rewrites to functions work differently than rewrites to other paths. They suggested that you could accomplish what you want by grabbing the incoming path (/.netlify/functions/netlify-support-test?foo=bar) directly in your function using event.path and then parsing the param you’re interested in within your function. Sorry for the confusion here. Let us know if that solution would work for you.