Backend FormWidget FileUpload does not validate correctly because of deferred binding?

I want to validate the dimensions of an uploaded image, using Backend\FormWidgets\FileUpload.

Uploading the image for the first time works just fine, but when I replace the first image with a new image the first image attributes are used for validation. I expect the new image attributes are used for validation.

It seems that validation does not work with deferred binding?

models/mymodel.php:

class MyModel extends Model {
    public $rules = ['image'=>'dimensions:min_width=1024'];
    public $attachOne = [
        'image' => \System\Models\File::class
    ];
}

models/mymodel/fields.yaml:

    image:
        type: fileupload
        mode: image

This sounds like a bug but would need to set up an example in the test plugin to verify it.