ContentResultService is executed instead of the injected one

I'm strugling with super weird issue. I wanted to test an example from

 https://world.episerver.com/documentation/developer-guides/content-delivery-api/how-to-customize-data-returned-to-clients

Unfortuantely my CustomContentResultService is not executed. Base ContentResultService is executed all the time. I've tried to debug 

ContentApiResult.cs becasue ContentResultService is called from there. In debuger 

 private readonly ContentResultService _contentResultService;

seems to be an instance of my CustomContenrService which is expected. But for some reason it's not executed. I've asked my colleague to do the same with the same codebase and everything is working for him. How is this possible? Is there any cache or anything which might cause this issue?