Role="tree" interpreted as role="outline", with warnings

Hi! I am trying to implement a basic tree role as described in WAI-ARIA.
A simple tree like this gets interpreted with role=“outline” in the Accessibility inspector, which is not a role specified in ARIA.

<ul role="tree" tabindex="0">
    <li role="treeitem" tabindex="-1" aria-level="1" aria-setinpos="1" aria-expanded="true">

The warning you is this (Interactive elements must be able to be activated using a keyboard):
You can clearly activate this element (using tab) so this is a false warning right?

Some screenshots from my browser inspection of a similar example:

This example at W3 also gives the same result and warning when inspected:

So two questions…

  1. Why is the tree role overwritten as an outline role (doesn’t exist in aria)?
  2. Why do I get this warning when it’s clearly not applicable?

Browser: Firefox 84.0.2
OS: MacOS Mojave 10.14.6