Mock inner Flurl HTTP client calls in REST API for testing

Question

The goal is to test REST API that makes another http calls inside by mocking these inner calls.

There are 2 Flurl HTTP clients:

1. One is in an integration test that makes calls to the REST API - testHttpClient
2. And one is in REST API that makes calls to another API - innerHttpClient

I was trying to mock the calls in the tests according to the Flurl HTTP documentation. Unfortunately, it seems only calls from testHttpClient are mocked. Setup rules are not working for innerHttpClient.

Any recommendations?