Filtered bezier curve not shown if all points are on the same height or width

Question

I got a path of an svg and am filtering it with an svg filter.

It works as long as all points are not on the same line

https://jsfiddle.net/vt6z7rp9/2/

  <path d="M 20 160 Q 180 40 280 160" class="pathStyle" stroke-width=2 />

  <path d="M 20 160 Q 180 120 280 160" class="pathStyle" stroke-width=2 />

  <!--why is this not shown?-->
  <path d="M 20 160 Q 180 160 280 160" class="pathStyle" stroke-width=2 />

I would expect that the 3rd path in the jsfiddle would also be displayed