This is a very strange issue, very rarely encountered, but not easily fixed.
When you are in this specific set of conditions, any export you make from Excel will create an image with a transparent bottom border (see picture below)
Before going into the necessary steps to reproduce this, lets see how to fix it:
Simply sign out of your windows session and sign back in.
When does this happen:
You encounter this bug (for some specific hardware configurations) when you connect a second screen to your computer, make it the principal display, and unplug it.
Unplugging makes the first display the main one, but Ms Office keep reference to the old one to compute the enhanced meta file used to export.