What is the return type for the image field?
If it’s array, there is an error but it’s printed into the inline styles so you can’t actually see it on the screen! So it’s kind of sneaky.
To solve that you need to do <div class="fill" style="width: 100px; height: 100px; background-image: url(<?php echo get_sub_field('image')["sizes"]["full"] ?>);"></div> to get the image url from the array.
