We can now agree that ACF is correctly loading the value, the problem you originally posted must be that you have a plugin that is modifying the pre_get_posts filter and preventing ACF from loading the gallery images.
Perhaps you can start debugging in the relationship field file. There is a function for format_value_for_api` – this is where the posts are loaded in from the array of IDS.
This will be a good place to start.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users