How to automatically enable custom worker dynos on Heroku specified in Procfile and app.json?

Question

I'm attempting to deploy our app using heroku review apps but we can't seem to enable the worker dynos automatically. It seems like the formation designations are being completely ignored. When I run curl -n https://api.heroku.com/apps/{APP_NAME}/formation \ -H "Accept: application/vnd.heroku+json; version=3" it returns the right number of processes, right process names, but with size hobby and quantity 0 instead of the sizes/quantities specified in app.json.

This is roughly what our Procfile and app.json configurations look like:

Procfile:

web: bundle exec puma -C config/puma.rb
worker-one: bundle exec sidekiq -c 1 -q something
worker-two: bundle exec sidekiq -c 10 -q something-else
release: bin/heroku_release_phase

app.json:

{
  "name": "Flowspace",
  "buildpacks": [
    {
      "url": "https://github.com/heroku/heroku-buildpack-multi-procfile"
    },
    {
      "url": "https://github.com/heroku/heroku-buildpack-ruby"
    },
    {
      "url": "https://github.com/Lostmyname/heroku-buildpack-post-build-clean"
    }
  ],
  "formation": {
    "web": {
      "quantity": 1,
      "size": "hobby"
    },
    "worker-one": {
      "quantity": 1,
      "size": "standard-1x"
    },
    "worker-two": {
      "quantity": 1,
      "size": "standard-1x"
    }
  },
  ...
}