I’m experiencing a vexing, hard to replicate problem. I use ACF on a custom post type to set a non-Featured Image. Occasionally when I save a post, the associated image’s post type will change from “attachment” to the custom post type it is associated with. It also makes the image post a child of the custom post. This then makes the image unusable across the entire site—including other posts unassociated with the custom post, but which use the image.
I have not been able to determine what factors trigger this bug.
ACF support said that they’ve seen this happen when using ACF in conjunction with Post Type Switcher—which I had installed. I’m going to disable that, and only reenable it when I really need it (rarely).