Getting confusing error for Identity Toolkit mfaEnrollment.start request

Question

After following the docs here, I am getting the below error. This error does not make sense as I am using the exact same access token creation process as I am for GIP tenant creation and it works just fine there.

Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential.

My body looks like this

def build_body(params)
 {
    :idToken => params[:id_token],
    :tenantId => ::ENV.fetch("DEFAULT_SHARED_GIP_TENANT"),
    :phoneEnrollmentInfo => {
      :phoneNumber => params[:phone_number],
      :recaptchaToken => params[:recaptcha_token]
    }
 }.to_json
end

My enroll call:

def enroll(body)
  access_token = ::Actions::Gip::AccessToken.new.call.success
  ::Net::HTTP.post(
    URI("https://identitytoolkit.googleapis.com/v2/accounts/mfaEnrollment:start"),
    body,
    { :Authorization => "Bearer #{access_token}", "Content-Type" => "application/json" }
  )
end

My access token action:

def fetch_token(json_string)
  ::Rails.cache.fetch("portal.gip.access_token", :expires_in => 55.minutes) do
    scope = ["https://www.googleapis.com/auth/identitytoolkit", "https://www.googleapis.com/auth/cloud-platform"]
    authorizer = Google::Auth::ServiceAccountCredentials.make_creds(
      :json_key_io => StringIO.new(json_string), # Service account creds
      :scope => scope
    )
    token = authorizer.fetch_access_token!
    token["access_token"]
  end
end

The service account has the roles Firebase Admin, Identity Platform Admin, and Identity Toolkit Admin