“_wp_attachment_metadata” not being written sometimes

WP 2.5 changed the way thumbnails files are named, so I’m changing a plugin so it gets the correct filename from the database, instead of just referencing the ‘.thumbnail’ file.

The problem is that, in my tests, when I upload an image, WP doesn’t seem to be consistent regarding the writing of the field “_wp_attachment_metadata” on the table “wp_postmeta”. Sometimes it does, sometimes it doesn’t and sometimes it just records “a:0:{}”.

Does anyone know in which moment WP does this writing? Is it when I upload or when I add the image to a post? Is this a known issue?

Thanks.