Netlify Dev issues a 301 redirect when a rewrite is specified

[ryaninvents]

Describe the bug

When running netlify dev locally, visiting a path that I have proxied results in a 301 redirect to a path beginning with .netlify/functions/.

To Reproduce

Steps to reproduce the behavior:

1. Write a Netlify function
2. Create a redirect rule in _redirects proxying to your function; for example:
   / /.netlify/functions/index 200
3. Run netlify dev
4. Visit the locally-running application in your browser
5. See that the URL has changed to begin with /.netlify/functions/ instead of the requested URL

Configuration

• netlify.toml

[build]
  command = 'yarn build'
  publish = 'build'
  functions = 'build-functions'
[dev]
  publish = 'build'
  functions = 'src/functions'

• Please enter the following command in a terminal and copy/paste its output:

% npx envinfo --system --binaries --npmPackages netlify-cli --npmGlobalPackages netlify-cli

  System:
    OS: macOS 11.0.1
    CPU: (8) x64 Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz
    Memory: 131.76 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 12.18.2 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.5 - /usr/local/bin/npm

% yarn why netlify-cli   
yarn why v1.22.4
[1/4] 🤔  Why do we have the module "netlify-cli"...?
[2/4] 🚚  Initialising dependency graph...
[3/4] 🔍  Finding dependency...
[4/4] 🚡  Calculating file sizes...
=> Found "netlify-cli@2.71.0"
info Reasons this module exists
   - "_project_#app" depends on it
   - Hoisted from "_project_#app#netlify-cli"
info Disk size without dependencies: "1.9MB"
info Disk size with unique dependencies: "31.63MB"
info Disk size with transitive dependencies: "244.82MB"
info Number of shared dependencies: 262
✨  Done in 1.49s.

Expected behavior

Netlify Dev should render the output of my index function and return 200 instead of returning the 301.

[ryaninvents]

Can't reproduce this any more, but I'm not sure what I changed.