Generate https image URL
Doing a quick test, works fine with HTTPS



Views REST XML



Views REST Json



All of the above is using FULL URL



Perhaps someone set it to Absolute file path, which will give you /path/image.jpg (with out the $base_url). Then in some template file they did http://example.com{{ field.image.content }}

update

using template_preprocess_image_formatter() is another solution to change the template formatter of the image.