I actually just revised my code a little. The field to receive the preview data needs to be in the template. I built a repeater with ACF to register the blocks. The acf field doesn’t even need to exist within the block fields, weird but it works.
'example' => array( 'attributes' => array( 'mode' => 'preview', 'data' => array( 'gutenberg_preview' => __('<img src="https://via.placeholder.com/680x720">'), ), ) )
Then in the template…
<div data="gutenberg-preview-img"><?php the_field('gutenberg_preview'); ?></div>
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