In my case the problem was the same name of the default webpack websocket and the mask in the proxy (/ws). Adding a new segment to the mask helped and instead of “/ws/**” I got “ws/api/**”.

{
    context: [
        "/ws/api/**",
        "/stream/ws/**"
    ],
    target: "wss://my-instance.com",
    ws: true,
    secure: false,
    changeOrigin: true,
},