This seems to be a bug with ruTorrent or rTorrent and has existed for more than a decade.
The workaround is to restart rTorrent, and after restart "added" time should appear on new torrents as expected.
Torrents already added won't get their "added" time updated, but new ones will get it.
To update for old torrents, "edit torrent", then press "OK" for changes. No need to change anything but this fixes the empty "added" time.
You can also try to sort by "Finished" column for time based sorting.