When this happens, and it happens with reasonable frequency, that photo will not fully download to any of my iOS devices. The affected photo cannot be edited on any iOS device since the full resolution download from iCloud fails. An additional symptom is that the download stalls at 25% progress in the Photos browser. The image will, however, download in full resolution via the iCloud Photos web app, and can be edited with Photos for Mac OS X.
I've tried deleting and restoring the photo with Photos on iOS, but that does not fix the photo. I have also tried waiting days to see if some iCloud maintenance task comes along and fixes it. It does not.
The only fix for me has been to edit the photo with Photos for Mac OS X and save it back to the iCloud Photo Library. The photo will then download to all of my iOS devices.
Photos may be out of Beta, but back up your images to be safe. (I keep full resolution images on my iMac.)