Hmm strange, I just tested the same thing in a fresh install and there it works as intended!
I do not see any errors in the dev console. I also tried to disable all plugins, including WPML which I thought that may cause issues, but it wasn't that either.
So I can't exactly tell what causes the problem, but as it works on a fresh install, the problem will probably lie somewhere within the theme.
The only thing that may help identify the problem is, that it does only take the images from the latest added group. So if I would have added 10 groups with images and I add the 11th, it will use the images from the 10th group.
I will try to do more testing, but the plugin doesn't seem to be the problem at least!