mcsRebuildEM - Triage - Failed to open compressed data file

Background
Continuing from slack: https://mariadb.slack.com/archives/C03G6V1RH1D/p1653333586247559

I tried using mcsRebuildEM with a client on prem because of their existing extent map not working nor their backups. Roman has some details on why the existing map didnt work, something about OID 1001 and only 79 bytes existing when 80 expected. In the end we had to wipe all their columnstore data and reinstall columnstore and re-import their data to get them back online which is not acceptable or even possible sometimes.

Ask
1) Whats wrong with `/var/lib/columnstore/data3/old_3/000.dir/010.dir/140.dir/014.dir/000.dir/FILE002.cdf`
2) why couldnt mcsRebuildEM open the file
3) what are some possible reasons the client environment / files get in this state ( guesses ok )

Files
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