The new "deselect" in Excel is a rushed and worthy of a high school "learn to code" class

It's not quite clear what you want to happen here, but I went through a number of different situations when trying out the new deselect feature and here's the supposed rules it seems to follow:

1.  If selections overlap:  Apply borders by order of selection but clear any borders applied on any previous selection before applying borders.  For "outside borders" this has the effect of overlapping pieces of paper.  You see all of the border of the most recent selection, and less of the previous selections as if they were laid on top of each other.

2.  Deselect cells:  When cells are deselected, new independent selection ranges must be defined so each range is contiguous.  So the top rows are one selection, bottom rows a second selection, and the conjoining columns on each side are their own selections, leaving a cap of unselected cells in the center.  When applying an outside border to this, it will apply the border to each selection independently.

3.  Adding smaller selections inside an existing selection:  Not possible.  Any attempt to select a range entirely inside of an existing range will result in deselecting those cells (ctrl+click) as in #2, changing the size of the original selection (shift+click), or selecting a new range (click).  Selecting the inside box first and then the outside box second will result in #1 and only the outside box's border will appear (as if laying a larger piece of paper over a smaller one).

The only way to accomplish #3 is to apply borders in separate steps and making a new selection before applying the second border.  Select any non-overlapping ranges first to apply the border, then make a new selection to border around all of them.