[Cerrado] Images not showing in Advanced translation

Background of the issue:
I am trying to translate images from a gallery in a custom post type using WPML's Advanced Translator. The issue is related to this page among others, but can not be seen outside translation vindow: enlace oculto. I have a custom post type with various custom meta fields, including a gallery. Usually, I can translate the title and alt text of images.

Symptoms:
Sometimes none of the images from the original language show up while I’m translating. The images still appear on the actual page when I visit it in the translated language. On some pages, only the featured image is displayed, and it’s often duplicated. Instead, I got no images in the translation window.

Questions:
Why are images not showing in the Advanced Translator?
Why is the featured image duplicated on some pages?