IPad – Why is iTunes not syncing photos from library after a certain date into the iOS device

I have an iPad mini (1st gen) I want to give it to a family member, so I operated a full system backup and reset. I have all the photos from the iPad inside of my library on OSX (in the Photos app).

I now want to put all these photos back into the iPad, so I did this (selected "Copy photos from photos" and checked "all the photos" with "include video") and synced:

However, there are more than 1.808 photos/videos in my library. Indeed, this is what my library looks like on Mac OSX. Please note the presence of items after the date "21 set 2015":

All the items after "21 set 2015" are not synced. This is what the photos app looks like on the iPad:

Why is this happening? What can I do to automatically sync photos which are newer than 21 september 2015?