This is actually not a bug, but rather the intended behavior; ID3 tag parsing is a rather slow process when done in aggregate, so Centova Cast will ignore media files which it has already indexed for the sake of performance.  Thus, when you re-index your media library, only tracks which have been added or removed will be processed.

We determined during testing that our clients preferred this behavior as opposed to having to wait for Centova Cast to re-index their entire media libraries (sometimes composed of tens or hundreds of gigabytes of media) every time they added one or two songs.

If you wish to have a track reindexed, click the "Update Media Library" link and wait for it to complete.  When it does, a "Perform full media library update" button will be displayed; click it to re-index all of your MP3s and pull in any changes.