acf_get_attachment forcefully triggers image_downsize (generating all derivatives) for *all* defined sizes

Let's assume we have the following sizes:

wp eval "print_r(wp_get_additional_image_sizes());"

Array
(
    [thumbnail] => Array
        (
            [width] => 500
            [height] => 0
            [crop] => 
        )

    [medium] => Array
        (
            [width] => 300
            [height] => 0
            [crop] => 
        )

    [large] => Array
        (
            [width] => 1024
            [height] => 0
            [crop] => 
        )

    [content-default] => Array
        (
            [width] => 3000
            [height] => 0
            [crop] => 
        )

    [content-blog] => Array
        (
            [width] => 3000
            [height] => 0
            [crop] => 
        )

and that we use lazy image generation (according to srcset). In our case Timber + Timmy.
At some point we use get_sub_field, before rendering the template, but it happens that get_sub_field will trigger the generation of all derivative for that image (even for sizes which were not set for this image).

Let's the attachment for the template which triggered get_sub_field is only even shown in thumbnail or content-default. Using ACF will instead generate 5 different derivative which is:

• useless in term of disk-space
• heavy and prejudicial in term of page load
• dangerous (It's easy to exceed time limit if you have 10 image sizes and 10 images on the front page) : 100 derivative must be generated, even though only 20 or 25 unique tuples actually exist

Here is a typical backtrace:

 get_sub_field              # In function.php, population the Timber context for example
 get_sub_field_object
 acf_format_value
 apply_filters('acf/format_value')
 WP_Hook->apply_filters
 _acf_apply_hook_variations
 apply_filters_ref_array('acf/format_value/type=image')
 WP_Hook->apply_filters
 acf_field_image->format_value
 acf_get_attachment              # <- this contains the fatal `foreach` loop.
 wp_get_attachment_image_src   # Called with all possible `size` argument
 image_downsize                   # Heavy Imagick operation (resize/crop/...)

The problem happens in acf_get_attachment which only take an attachment as argument (with no context information) and blindly get_intermediate_image_sizes() (all image sizes) and call wp_get_attachment_image_src which triggers image_downsize.

I don't have a clear overview of the whole process and what would be the correct way to fix this. But as a first solution that comes to mind : context could be provided down to acf_get_attachment to selectively provide sizes.

I actually fixed my problem by identifying these images ACF and loading them using new TimberImage(get_sub_field($name, false)) which doesn't expose such a behavior ($format_value = false).
But I think that's a behavior worth being documented (or maybe even fixed) because of its potential to disturb configuration relying on multiple image sizes.

Start with the acf_get_attachment call in the supplied backtrace, then follow acf_field_image->format_value and wp_get_attachment_image_src through the image-size loop. Reproduce the get_sub_field case with the listed image sizes and verify that formatting no longer generates every derivative when only selected sizes are used.