Not sure if it's a bug or an oversight, but when you upload photos into a mirrored forum thread, the metadata isn't shown in the lightbox sidebar. If I upload the same image directly to the relevant media gallery, the photo's metadata shows up in the sidebar when I view it from the media...
If the image was uploaded before this issue was fixed then it won't have the metadata unless it is re-uploaded.
You can turn off the lightbox if you'd prefer to get the old functionality back for the time being. Mirrored attachments will still use the lightbox in threads but everywhere else will just navigate straight to the media.