This has been a long standing issue.
The date added tag is not a common tag, so reload tag won't help you here because it's not stored in the media file's ID3 tags.
The date added in Rekordbox seems to follow the date notation as specified in the system preferences language. Change that for whatever reason and you'll lose sorting capabilities.
If you import tracks through different sources (e.g. drag 'n drop tracks, use an xml file to import tracks that has a date added field and iTunes import) the date added column will be messed up.
if a track is already part of the Rekordbox collection, the date added field will always be ignored if imported again. Regardless of where it is being imported from.
So if the date added was blank at first for whatever reason, it will remain blank. Needless to say it's not editable in the Rekordbox tag editor.
The Rekordbox team is aware of this (mis)behaviour, but I'm afraid it's not high on their priority list.