I am also seeing this behavior, and here's what I've discovered with some testing:
Import a JPG file which doesn't contain any IPTC info.
Add Copyright & Creator fields within Aperture, either at import or afterwords.
Export the image as JPG.
View the exported image in Preview. The Copyright & Creator fields are apparently missing when you look at the file info.
So far, this confirms what you have seen.
But...
If you view the file info in other applications, the data is actually present. For example:
- In PhotoMechanic the field's data is present. Further, if you change any metadata in PM and resave the image, then the "missing" metadata now shows up in Preview.
- In Photoshop, the fields contain blanks, but apparently an incorrect number of blanks. So PS is a bit confused, obviously. And like PM, if you resave the image from Photoshop then the IPTC data shows up properly in Preview.
- Another viewer-only application show the fields correctly.
Via testing, I only see this anomaly with my Nikon D3; other camera's JPG images do not exhibit the "missing" data in Preview after export. As you observed, RAW files are not affected.
Conclusion:
- Aperture screws up certain metadata when exporting original JPG images, but only if you view the metadata in Preview, and then only for specific cameras.
- The data is actually present, but some software can't see it properly.
Please pass along any findings if you can confirm what I've discovered. Since Aperture and Preview are both Apple applications, one would think they could get on the same page. Let's hope.
Edit: Added note about saving from PS.