Second issue reported in the comment was caused by some shapekeys
flagged as embedded liboverride data, when their owner ID (Mesh e.g.)
is not a liboverride at all.
The `BKE_lib_override_library_validate` called on file read was only
covering real liboverride IDs, which where then supposed to take care of
their own embedded data. But since the owner ID is already a full
non-override ID, its embedded data ended up never being properly
sanitized.
Most likely more corruption data from quite old files and/or deprecated
experimental features.
da101d8cdb