route_spec.rb

describe 'something' do
  let(:route) { instance_double('Net::IP::Route', 'route') }
  before(:each) do
    allow(Net::IP::Route).to receive(:new).with({ test: 'data' }).and_return(route)
  end
  
  it 'doesn't suck' do
    expect(Net::IP).to receive(:routes).with(route)
    provider.create(context, name, {input: 'foo'})
  end
end

I suspect the allows not to match. change them to an expect temporarily?

Also double check, maybe even pry into, that your code is doing the right thing. in long debugging sessions things can get lost

Switching to expect results in the same error.

Using pry, I don't see anything out of the ordinary, but seeing as puppet_munge is called by the create method, is allow(provider).to receive(:puppet_munge).with('should').and_return('munged') even the correct way to mock the results of that method?

none of the allows/expects are failing, it's failing specifically on provider.create() in the test. Almost like that line partially ignores the allows

Method being tested for reference purposes: https://github.com/dhollinger/puppet-network/blob/resource_refactor/lib/puppet/provider/network_route/network_route.rb#L78

To wrap up this thread, after some online debugging we found a missing stub, causing this.