Importable assets missing source info

When I run a python script to gather the source info for importable assets (textures, meshes, skeletal meshes…) some of these assets return either a empty string for RelativeFilename or an empty list.

example results:
[{ “RelativeFilename” : “”, “Timestamp” : “-2006054656”, “FileMD5” : “” }]

This results in the “re-import” option becoming unavailable.

I was wondering if anyone knows what kind of circumstances cause this information to go missing?