After doing a "reload tags" for all files in my Rekordbox collection, the "Release Date" for these songs is no longer showing for ALL mp3 files in my collection. (The Release Date /is/ still visible for the .aif files in my collection.)
I can reproduce the problem by importing one of the mp3 files from a backup into Rekordbox again. The release date is then visible in both the track view and the info pane. The select boxes are properly loaded with the date. However, if I do a "reload tag" on that mp3 file, the date disappears from both locations. If I remove the mp3 file from the collection and add it again, the Release Date does not show up. I hope this does not mean that I've lost this metadata for my ENTIRE library.
If I do an update tags on .aif files, the release date appears unharmed. Only removed for mp3 files. I experienced this problem using Rekordbox 4.1 and subsequently updated to 4.1.1 and still see the same behavior. These were standard mp3s purchased at Beatport as well as other sources.