Why is feDropShadow poorly supported in non-browsers and leads to things disappear?

Question

The following happens to me on a regular basis when trying to write SVGs manually.

• I perform a web search for "svg drop shadow", and amongst the first hits is the MDN example on feDropShadow, which feels a decent reference.
• I copy/paste that example into a local .svg file.
• When I look at this local .svg with various document viewers it shows as a blank white page -- not at all what the example is supposed to look like, and I'm very confused.

The source of the example looks like that:

<svg viewBox="0 0 30 10" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <filter id="shadow">
      <feDropShadow dx="0.2" dy="0.4" stdDeviation="0.2" />
    </filter>
    <filter id="shadow2">
      <feDropShadow dx="0" dy="0" stdDeviation="0.5" flood-color="cyan" />
    </filter>
    <filter id="shadow3">
      <feDropShadow
        dx="-0.8"
        dy="-0.8"
        stdDeviation="0"
        flood-color="pink"
        flood-opacity="0.5" />
    </filter>
  </defs>

  <circle cx="5" cy="50%" r="4" style="fill:pink; filter:url(#shadow);" />

  <circle cx="15" cy="50%" r="4" style="fill:pink; filter:url(#shadow2);" />

  <circle cx="25" cy="50%" r="4" style="fill:pink; filter:url(#shadow3);" />
</svg>

and should look like this:

I'm a Linux user and the example renders as a blank white page in basically all my local tooling including:

• All my image viewers I tried including eog (Ubuntu default image viewer) and geeqie.
• Inkspace
• ImageMagick's convert

What adds to my confusion is that they don't render anything all, i.e., they seem to even ignore the "fill" as a result of feDropShadow being there. For instance, this is what I see in Inkscape (I've selected the elements to show that they are there in principle, just not rendering at all):

I can also use Inkscapes filter editor and remove the feDropShadow filters, which makes the elements appear again:

On the other hand the file does render correctly in any browser I tried. Also, the browser support matrix on MDN notes that most browsers have added support ~10 years ago, suggesting that it is not a brand new feature of the SVG standard.

I'm wondering why this is the case.

• Is there something unusual about the MDN example that could explain why it only works in browsers, or is it just a coincidence that all my local renderers simply don't support feDropShadow?
• Is it expected behavior that the renderers do to not show the elements at all if they don't support a certain filter, or is this yet another coincidence that they all end up showing a blank page?

(I couldn't identify the underlying rendering engine for all of them, so it also may come down to different tools simply using the same underlying renderer, but if I remember correctly at least Inkscape and ImageMagick have different renderers...)