+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.
ACF 4.4.0
WP 4.1.1