If a user deletes an image from the image library, then the image field returns blank. It should say, error: image deleted/not found and give the option to use a different image. However, I had to change the custom field type to text, remove the image ID which appeared, then switch the custom field back to image.
+1 to this bug and just saw it start today. I deleted some images from the media library then went to page to pick a new image and only saw the field title. This is happening on all custom fields for images.
I also was able to enlarge the img manually with the web inspector by altering the selector “.acf-image-uploader img” and setting height and width to something like 150px, then hovering over the space where the image would show.