The only thing it didn't pick up correctly was 'Welcome to Demon School! Iruma-kun' (https://www.themoviedb.org/tv/91801?language=en-US). I have everything named like ./Lucifer/Season 1/Lucifer - S01E06 - Favorite Son (HDTV-1080p).mkv
and for the one that was missed it's Welcome to Demon School! Iruma-kun/Season 1/Welcome to Demon School! Iruma-kun - S01E01 - Iruma-kun from Demon School (WEBDL-1080p).mkv
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I tried playing back a UHD rip (Game of Thrones) and it played back fine for 5 minutes or so until I got bored (with the experimental player on the Apple TV 4). Same with the normal version, they all played good. My only initial improvement suggestions would be what you're probably already tracking (multi-user support/multi-version support/ignore 'The' sorting/etc).
Really great for an Alpha!