Edits or filters that you apply to a photo on your iOS device are stored in the metadata included with the photo. The raw photo, which includes the metadata, is then uploaded to the Dropbox servers. However, if you attempt to view the photo using an app or software that can't interpret the new metadata, the original photo (without modifications) will be shown instead.
When uploading, the Dropbox application is given a thumbnail of your photo with the modification applied, so you may see the edited thumbnail of your photo during the automatic Camera Upload process, or in your Photos tab for manual uploads.
Your photo edits have not been deleted, and will be viewable on devices with software that can read the metadata.