This is also filed as FB9706029. Thanks for looking into it!

It should work when you remove the -I $MTL_HEADER_SEARCH_PATHS part from your custom build rule. Though it is mentioned in the WWDC video, it actually causes problems when MTL_HEADER_SEARCH_PATHS is empty. See this answer to a related question. Usually, you don't need that parameter if you don't have a complicated file graph or external dependencies.

Hmm, interesting. I wonder if the filter is not designed to be re-used. Can you please try with rawFilter.previewImage instead of outputImage?

Also filed as FB9498614.