It seems that this behavior is related to the minimatica theme. There is a function named minimatica_paged_posts that modifies the posts_per_page value before the gallery is displayed. It targets posts and pages in its original form, though it appears that it should just target posts. The function can be easily modified directly in the theme's functions.php file or overridden through the use of a child theme.
If you want to take the direct approach, you would edit line 137 of the functions.php file, in the minimatica theme directory, to insert a conditional statement to check whether a page was being viewed. You would change it from this:
) && ( ! is_single() )
) && ( ! is_single() && ! is_page() )
If you would prefer to use a child theme instead, we have set one up which includes this function override. You can download the child theme from our site if you are interested in using it. The nice thing about this approach is that you will not need to edit any files and your changes will remain in place even if you update the parent minimatica theme.
We hope that this helps to address your question. Let us know if we can be of further assistance with RPS Image Gallery.