Just missing the Initialized notification, which comes immediately after the initialize response is received.

{
  "jsonrpc": "2.0",
  "method": "initialized",
  "params": {}
}

Incidentally the idea that sending and receiving notifications 1 at a time is always the case is a mistake, e.g.

src: https://iwanabethatguy.github.io/language-server-protocol-inspector/