The download monitor blocks post-processing with a bytes-incomplete guard:
if size > 0 and transferred < size: continue
_stream_to_file throttles engine updates to every 0.5s. The last tick before
the file finishes typically leaves transferred slightly below the Content-Length
size in the engine record. Other streaming clients (YouTube, Tidal, HiFi, etc.)
use their own download threads and don't track bytes at all, so size stays 0
and the guard is always skipped. Amazon was the only client hitting it.
Fix: just before returning the file path from _download_sync, write a final
engine record update setting size == transferred == out_path.stat().st_size
(the decrypted output size). The bytes-incomplete guard then sees
transferred == size and falls through to trigger post-processing normally.
Broque Thomas