WebRTC lets browsers talk to each other directly for calls and screen sharing. To set that up, the browser gathers its network addresses, and a page can ask to see them. Those connections do not always go through the proxy.
Antidetect browsers handle this per profile, usually with a setting that makes WebRTC report the proxy's address. In an ordinary browser it can be limited with an extension or a setting. After changing it, open a leak-test page through the proxy and check that only the proxy's address appears.
Read next: How to test a proxy