Official comment
That detail about the trimming is not accurate anymore. We will change that. The trimming is now a concave hull. This has been changed. Thank you for bringing it to our attention.
The trimming polygon is calculated using average spacing and height and camera FOV. It might be a little oversized for your map area but the automated trimming is intended to remove the far out areas that are prone to high error. It looks like the sides of your most recent map got trimmed but not the tops. This is not uncommon.
From the Map Trimming article it shows that you can manually trim the areas you don't want using the Polygon Tool and the switch to enable the trimming at the top of the Map Detail page.
The Location Map trimming is still something we are evaluating.
