To have all the pictures showing inside the Pix-Star Snap app you need to make sure you have the following configuration activated:
of the iPhone / iPad then to:
Photos and Camera
then inside in the first section related to " iCloud Photo Library " make sure you have this selected:
Download and Keep Originals
DO NOT select:
Optimize iPhone Storage
same as on the picture below:
Now you should see all your pictures in the Pix-Star Snap app!
Many thanks to Ken S. for finding this and reporting the solution to us.