The simple answer in this case was that there are overrides:
- HttpContext.Response.AddCacheItemDependency
- HttpContext.Response.AddCacheItemDependencies
Recommend adding a note to this page:
https://docs.xperience.io/configuring-xperience/configuring-caching/caching-on-mvc-sites#CachingonMVCsites-Cachingtheoutputofcontrolleractions