Is it possible to access the app data of third party apps that aren't listed under File Sharing in iCloud?
You could try backing up the iPhone in iTunes and dig into the backup locally to see if the photo files are in the app's backed up data. I work on a tool that can help you dig into the backup files, Decipher Backup Browser: https://deciphertools.com/decipher-backup-browser.html

You could either look through the app's data in all files view, or try the all photos gallery view in DBB. Or if you want to contact me directly about searching around the backup, there's a contact link at the top of all of our pages.