You can’t download an object that is just System.FileDocument, you’ll need to create a specialization in your module, define entity access for your user role(s), use that entity in your create Report action and download that. See also the documentation on this topic.
have you tried setting the filename directly? I have tested simple pdf with title and it works.
I am running into this same issue and have my specialization of System.FileDocument configured correctly. The odd thing is, I only run into this issue in our production build and not in my local environment. The flow being used, as well as the objects, are identical.
Has anyone else figured out how to resolve this?