Many cameras are not supported by Photos and Apple’s decoder. I have a special (unique) way of extending Apple’s decoder, but it only works in Nitro.
So, if you view an image that is not supported by Photos, you usually get a blank thumbnail, and an error in Edit. However, Nitro can both display the thumbnail and edit.
When you save the edits, Nitro saves a new JPEG into the library with the edits. That you can see in Photos.
You may want to read this article I wrote some time ago. It explains some of this.