Our customer added documents when portal had default language en_GB. After changing default portal language en_US extracted metadata field is empty.
1. Control Panel -> Portal Settings -> Display Settings
(remove en_US from Current and set en_GB as default language)
2. Go to document libraray and add basic document (you should see extraced metadata when preview the document)
3. Control Panel -> Portal Settings -> Display Settings
(remove en_GB from Current and set en_US as default language)
4. Go to preview of the same document (the box with metadata is empty)
There might be similar risk when custom metadata sets are created with default language and then portal language changes.