It appears that there is a bug when the border standoffs aren't equal.

If you set the left and right to the same space you don't have a problem.
If left is greater than right, you see this white space
If right is greater than left, the shading goes beyond the border

Simple fix is to make the left/right sizes the same. If you are unhappy with that fix, put the text inside a table cell and shade the table.

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Andrew Lockton
Chrysalis Design, Melbourne Australia