It seems uBlock Origin has a setting (right on the main settings page) that can prevent the local IP to be leaked: https://github.com/gorhill/uBlock/wiki/Prevent-WebRTC-from-l...

You can test it here: https://browserleaks.com/webrtc

I tried that test page with the mentioned setting 1) enabled and 2) disabled. I did not see my local IP on 1), I did get see it reported with 2) - so it seems the block works. NOTE: Whether uBlock Origin was enabled for the specific page did not matter, the WebRTC block setting seems to be global and independent of whether the extension blocks anything else on the site.