This is easy to reproduce with the following example:
  <style>#foo { outline-offset: 10px; }</style>
  <div id="foo"></div>
  <script>console.log(getComputedStyle(foo).outlineOffset);</script>

It should log 10px but it actually logs 0px.

This is probably a mistake as "outline-width" is special on the spec (https://drafts.csswg.org/css-ui/#outline-props).
For "outline-width" the computed value says: "absolute length; 0 if the outline style is none."
But for "outline-offset" it says: "absolute length"

There's a check causing this behavior at RenderStyle::outlineOffset().