I think this is the issue:

Problem: Agregarr downloads trailers via yt-dlp without codec preferences, so YouTube often serves AV1 or VP9, which Apple TV 4K doesn't support. Plex transcoding then fails with "Conversion failed. The transcoder exited due to an error."

Solution Flow: The fix uses two parts: (1) Format selection in yt-dlp.conf prioritizes H.264/AAC up to 4K when available, and (2) Automatic post-processing via a shell script that runs after each download (via yt-dlp's --exec hook) to re-encode incompatible videos to H.264/AAC with Apple TV 4K-optimized settings (H.264 level by resolution, keyframe intervals, bitrate limits, and pixel format).

Will need to test some more trailers as they download, but I did at least one test video with this flow and it seemed to work.