I cannot reproduce the following example from the handbook:
When I try it, the hatching applies to the empty inner shapes of the letter as well. As such, whenever I apply the Filter to an “A” (or similar shapes), I receive two (or more) overlaying hatched areas.
This could be related to a path direction issue. As a first step, try correcting the path directions automatically with Path → Correct Path Directions and see if the filter works as expected then.