Fira Sans CDN Font does not support "tnum" feature

When Fira Sans font is included in a page using the CDN link, the `font-feature-settings: "tnum"` CSS property doesn't work.

Oddly enough, if I download the Fira Sans font from Google's site and install it locally, the feature setting works: https://fonts.google.com/specimen/Fira+Sans?selection.family=Fira+Sans
It also works if I link the font from Mozilla's CDN.

It's only when I link to Google's CDN and don't have the font installed locally. I've made a plnkr to demonstrate the issue: https://plnkr.co/edit/SnSm7siVrk5AiQq9NDUK?p=preview