The render looked wrong the moment it opened. A 360 video, which should have let me look around, was just a flat stretched rectangle.
The quality score said otherwise. The compression tool I trust measures how close the picture is to the original, and this one scored 96. The pixels were fine. The probe confirmed it: 5760 by 2880, intact. What was missing was a tiny piece of metadata that tells the player the video is spherical.
Finding the edges of it
The encoder had dropped the flag. I reproduced it on a five-second test clip, which told me it was a cause and not a fluke. Then I ran a header-only sweep across all 3,639 compressed files, which reads the first few bytes instead of the whole video. It found exactly 10 affected files.
The repair
I did not re-encode anything. I added the tag back and nothing else. The video stream checksum was the same before and after, each file grew by 487 bytes of metadata, and every date was kept. Then I put a guard in all eight compressor scripts so that any 360 source is copied as-is instead of being encoded.
The part I keep thinking about
A quality score is a very good answer to one question. It was never asked whether the file still behaved like a 360 video. It also did not know about flight telemetry tracks or spatial audio, which turned out to be at risk too.
The next thing I am building is a check that compares the metadata of a compressed file to its original before it is allowed to replace anything. Measure what you care about, and then measure what you forgot you cared about.