MPCook relies on global data that is accumulated during package cooks on CookWorkers be replicated to the CookDirector.
Some global data that is accumulated during package cooks is CookAdditionalFilesOverride. Theoretically this is supposed to be collected by the CookDirector by writing the files out to disk. But this has not been tested, and some middleware plugin providers have reported that their data is not being collected (it is present in SPCook but not in MPCook).
Add test content for CookAdditionalFilesOverride in EngineTest, add any missing replication for it that is necessary, and provide tools the middleware plugin providers can use to debug why their data is missing.
TODO: Create test data in EngineTest
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-198505 in the post.