Rabl not running for DELETE

I am in the process of upgrading an old Rails app. Just tried increasing Grape from version 0.17.0 to 0.19.2. I hit on issue with the rabl no longer being processed for DELETE.

My API looks like:

desc 'Disable to send push notification'
delete '/push_notifiable', rabl: 'rooms/push_notifiable' do
  @room = Room.find(params[:room_id])
  @push_notifiable = false
end

And the test that now fails looks a bit like:

describe 'DELETE /rooms/:room_id/push_notifiable' do
  subject { delete url, params: params }
  it 'returns 200 status, and correct json' do
    subject
    expect(response.status).to eq 200
    expect(json['room']['room_type']).to eq 'XXXX'
  end
end

The status test passes, but the response body is no longer the JSON generated by my rabl, just simply the string false.

Use the shown DELETE route with the rabl option and the failing DELETE /rooms/:room_id/push_notifiable test as the reproduction. Trace why the response body becomes the string false after the Grape upgrade; done means the request still returns status 200 and the JSON generated by the rooms/push_notifiable Rabl template.