Thanks again Shane.
I tried recreating it but it does not seem to help. One thing I have found is it works better with an Artwork that has more than 2 Detail View children. So I added a random third image here hidden link and it seems to work. Unfortunately most Artworks have 2 children.
I am attaching screenshots of the post types, view and layout it is in in case you can see something I am doing wrong.
Thank you for looking at this. I tried upgrading the site to PHP 7.0.6 but that did not seem to help and caused problems with the parametric slider View. I have downgraded to 5.6.21 but the problem persists. There are pagination dots now under the slideshow which show that the first image reappears as the second image and the second image then becomes the first (if that makes any sense).