This issue has been resolved.
As of June 19th, all recently maps uploaded to Minecraft World Map have failed to render. Under these circumstances, most maps will have a black box where an overview would normally be. If you already uploaded your map before this issue occurred, and you attempt to update it with a new archive, please be aware that the overview will not update and the Direct Download button may break.
I have contacted Skeight about this issue, and it should be resolved shortly.