No refresh token while authorizing Device Access

I am able to successfully authorize a user through Partner Connections Manager, but when I request tokens from https://www.googleapis.com/oauth2/v4/token using my authorization code, I do not receive a refresh_token in the response, only an access_token is present:

{
  access_token: 'my-access-token',
  expires_in: 3599,
  scope: 'https://www.googleapis.com/auth/sdm.service',
  token_type: 'Bearer'
}

Make sure to specify access_type=offline in your Partner Connections Manager (PCM) URL. Omitting it assumes access_type=online, which does not provide a refresh token.

For example, the PCM URL should look something like this, where access_type=offline:

https://nestservices.google.com/partnerconnections/project-id/auth?
  redirect_uri=my-redirect-uri&
  access_type=offline&
  prompt=consent&
  client_id=my-client-id&
  response_type=code&
  scope=https://www.googleapis.com/auth/sdm.service

Then, the subsequent token response from https://www.googleapis.com/oauth2/v4/token should have what you expect:

{
  "access_token": "my-access-token",
  "expires_in": 3599,
  "refresh_token": "my-refresh-token",
  "scope": "https://www.googleapis.com/auth/sdm.service",
  "token_type": "Bearer"
}

For more information, see Authorize an Account on the Device Access site.