Geospoof: a firefox add-on for convenient geolocation privacy

I just installed it. I see how it it works now. I have tested the three options you link to in the extension itself. I am however on Mullvad and while Time Zone and Geolocation is matching the location I chose, WebRTC page is showing my VPN location. Is this how it’s supposed to work or is it meant to provide obfuscation primarily when not on a VPN?