Downloading with transcoding doesn't work on both Android & iOS

Plex Players
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  1. Server 1.43.3.10861, Ubuntu LXC, Plex Pass active
  2. Fails on 2× Android + 1× iOS, every library item
  3. The log excerpt showing add→200, TranscodeUniversalRequest, PlayQueue added, then silence
  4. Activity → Conversion stays empty, no Plex Transcoder process spawns, no conversion log written, downloads temp dir writable, direct downloads and live transcoding both fine