"Keep in mind that this feature is to prevent leakage of your non-internet-facing IP adresses. The purpose of this feature is not to hide your current internet-facing IP address -- so be cautious to not misinterpret the results of some WebRTC-local-IP-address-leakage tests found online."

That said, my Firefox 91 and Safari don't leak local IPs regardless of the ublock setting.

Warrants more investigation perhaps.