[FFmpeg-user] "moov atom not found"

[bbb]

I have a few folders of old mp4 files that won't play anymore. All of 
them show the same error

$ ffprobe broken_file.mp4
ffprobe version 7.0-static ...
...
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x71f4a00] moov atom not found
broken_file.mp4: Invalid data found when processing input

So far, I've tried this:
$ ffmpeg -i broken_file.mp4 -map 0 -c:v copy -c:a copy -c:s copy 
-movflags faststart fixed_file.mp4
... moov atom not found ...

and this:
$ untrunc -a good_file.mp4 broken_file.mp4 -o fixed_file.mp4
...
Chunks 36629 times: 0
Track: 1 time: 512 nsamples: 1 tot: 0.04
Track: 0 time: 1024 nsamples: 1 tot: 0.0213333
Track: 1 time: 512 nsamples: 1 tot: 0.08
Track: 0 time: 2048 nsamples: 2 tot: 0.064
...
Track: 1 time: 512 nsamples: 1 tot: 4.04
Track: 0 time: 2048 nsamples: 2 tot: 4.01067
Repair: broken_file.mp4
Mdat not found!
Trying a different approach to locate mdat start
Repair: broken_file.mp4
1064 vs: 40
2296 vs: 248
3320 vs: 248
4344 vs: 248
...
73131256 vs: 248
73131972 vs: 196
Trying a different approach to locate mdat start
Repair: broken_file.mp4
Mdat not found!
Trying a different approach to locate mdat start
Repair: broken_file.mp4
Mdat not found!
Failed recovering the file
Saving to: fixed_file.mp4

I know for sure, that they used to play a long time ago. Of course, it's 
not completely impossible, but maybe a bit unlikely, that all of those 
files disintegrated on the file system. Could it be possible that this 
is the result of an outdated encoding or some hypothetical compression 
hack, that used to work a long time ago but doesn't work any more?
They were available for download on the internet around 10 years ago and 
they are very low quality and highly compressed. Each one around 
3MiB/min. I also have some slightly higher quality files from the same 
source (around 12MiB/min) that have been saved next to the others (on 
the same file system) and all of those are still playable.

This is not a high priority issue. Those files are completely 
dispensable. I'm just asking out of curiosity.