Imported files are regenerated but invisible in Media dashboard

I imported a bunch of files via WordPress importer tool over some existing media files. Old files are 308 in number, total files now are 740 with 3 missing errors so 737.

With Media dashboard, or Regenerate Thumbnails plugin I can only see the 308 old files.

With your plugin I can see all 740 files and I succesfully regenerated 737 files. Inspecting the folders and database shows everything in right condition. Howeever I can still see only 308 files in Media dashboard.

Do you have any suggestion about how to further debug this issue?

I even ran queries in both SQL and PHP. The standard wp_query call to show all media files also output only 308 results. SQL query with same where clause like wp_query against wp_posts table outputs 740 results.