other8026 dhhdjbd https://issues.chromium.org/issues/40061495. The demo that was shared here (https://niespodd.github.io/webrtc-local-ip-leak, a demo to show WebRTC can leak the local IP) has the same results for me. No local IP using Vanadium, but it shows up on Firefox Focus, same as with dnscheck.tools. I'm just going to mark this as solved because using a Chromium-based browser does the trick and it's NOT a GrapheneOS issue.
GrapheneOS Vanadium prevents WebRTC IP / mDNS leaks by default due to the default WebRTC IP privacy configuration.