Yet another bug in Illustrator. What happened to that dev team? I guess dumbing down the app for scrapbooking moms is hard work.
When I use Export As... to create PNGs defined by the Artboard, the preview window gives false information. Below is the art board defining the extent of the PNG:

Here is what the Export As... window looks like:

Here is the resulting preview window. Notice the big blank white space:

Yet the final product is as desired:

Unfortunately, before I discovered that the preview window was reflecting false information, I spent all kinds of time trying to figure out what was "wrong" with my files, like 3 hours or so, only to discover it was another bug. Thanks, Illustrator dev team!