To get back photos that got deleted on iPhone on its own, you can use iPhone Data Recovery software to recover your deleted photos directly from iPhone 6 without any backup. However, if you have created the backup of your iPhone 6 to iTunes or iCloud before your iPhone deleted your photos by itself, then you can restore your iPhone from iTunes or iCloud backup.
Once, you recover your photos, you can then “fix iPhone deleting photos by its own” issue. As you have said that your iPhone gives warning message that it does not have sufficient storage – may be your photos is getting deleted by its own due to this reason.
Hence, it is recommended that you must first transfer all the photos to computer or any other external storage devices and then free up space on your iPhone by deleting the photos. May be this can fix the issue of iPhone deleting photos by itself.