Trying to access array offset on value of type bool…in presswell-art-direction

I have begun using Presswell Art Directed images in my theme, however, I am getting inconsistent results with attempting to display them. I have added image sizes like so:

pwad_add_image_size( array(
'name' => 'Art-directed post thumbnail',
'key' => 'pwad-post-thumbnail',
'width' => '1920',
'height' => '1920',
'thumbnails' => array(
array(
'name' => 'Banner',
'key' => 'pwad-banner',
'width' => '1920',
'height' => '700',
),
array(
'name' => 'Hero',
'key' => 'pwad-hero',
'width' => '1920',
'height' => '846',
),
array(
'name' => 'Index thumbnail',
'key' => 'pwad-index-thumb',
'width' => '640',
'height' => '480',
),
),
) );

I have additional post images using ACF which are set to display via Image ID. In one instance, this works:

echo pwad_get_image( $image, 'pwad-post-thumbnail-pwad-banner' );

This works with the overload pattern only, not with the standard pattern.

On a different page, I receive the following error:

Warning: Trying to access array offset on value of type bool in?/Users/<my user>/Sites/<my site name>/app/public/wp-content/plugins/presswell-art-direction/presswell-art-direction.php?on line?557

var_dump on my $image reveals a string with a length of 0:

string(0) “”

That’s a clear indicator that the image is considered nonexistent, but I can’t figure out why PWAD isn’t producing a thumbnail of this image. Any idea as to why that might be?