Hi, I know this is older thread, but I ran into same problem and fixed it in a similar way @jeremynative did.
Addind a tag with class “fancybox-img” and actual url of full size img as href worked for me.
I was basically doing image swiper and in a loop when I was generating images I just added a tag mentioned above.
Here is my code (notice the a tag i added on img):
<div class="swiper-container gallery-swiper">
<div class="swiper-wrapper">
<?php
$gallery = get_field("galerie");
if ($gallery)
foreach ( $gallery as $img ){
?>
<div class="swiper-slide">
<a class="fancybox-img" href="<?php echo esc_url($img['url']);?>">
<img src="<?php echo esc_url($img['sizes']['homepage-nav']);?>" link=file alt="<?php esc_attr($img['alt']);?>"/>
</a>
</div>
<?php
}
?>
</div>
<div class="swiper-button-next"></div>
<div class="swiper-button-prev"></div>
</div>
Hope tihs will be helpfull to someone.