It is because the “series” in your case is not a post type, but a taxonomy – connecting to posts. The “Release year”, “Creators”, “Stars” etc.. are just supporting information for the actual taxonomy term, stored as term metadata. Term meta is similar to post meta, the difference is that the post meta stores extra “meta” information about posts, the term meta stores extra information about taxonomy term objects.
It is not possible to create filters based on these term metadata, only by the actual terms themselves.Best,
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