onRouteDidUpdate does not see new Head metadata

[slorber]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

When navigating between routes, the onRouteDidUpdate client module lifecycle does not see the newly updated Head / Helmet metadata

This leads to annoying usage of setTimeout in userland plugins.

As commented in #3399 (comment), I believe this is due to react-helmet-async using <Helmet defer={true}/> by default (to confirm).

We should study the impact of turning defer={false}: performance or anything else

Reproducible demo

No response

Steps to reproduce

Take a deploy preview, navigate to another page, and look at console logs.
https://deploy-preview-8275--docusaurus-2.netlify.app/

Expected behavior

title/description (and other metadata) should rather be correct

Actual behavior

title/description is incorrect

Your environment

No response

Self-service

• I'd be willing to fix this bug myself.