Bug: React 19 does not attach custom element event listeners during hydration

When hydrating server-rendered markup, React does not attach event listeners for custom element events that have been configured on the custom element using the React's on<custom event name> prop syntax. The event handler is only attached after the first client render, not during hydration. This issue affects all React SSR frameworks like Next.js (linking a related issue from Next.js: https://github.com/vercel/next.js/issues/84091 - note that the issue is on React side).

React version: 19.2.3

Steps To Reproduce

1. Open the CodeSandbox example.
2. Click the first "Emit custom event" button.
3. Click the second "Emit custom event" button.

Link to code example: https://codesandbox.io/p/devbox/thirsty-leaf-4n6r2t

Description of the demo

The CodeSandbox simulates SSR + hydration using only React:

• Server HTML is generated via renderToString
• The markup is injected into the DOM
• hydrateRoot is used to hydrate the content

A custom element <my-element>:

• Uses Shadow DOM
• Dispatches a CustomEvent("my-event") when clicking the "Emit custom event" button

Each React component (NotWorking & WorksWithForcedRerender) renders:

<my-element
  onmy-event={() => {
    console.log(
      "React custom event handler fired from <component name> component"
    );
    alert("React custom event handler fired from <component name> component");
  }}
/>

The current behavior

The onmy-event prop on <my-element> in <NotWorking> React component does not call the configured callback function and does not cause a native alert to pop up because React does properly attach event listener for the onmy-event prop during hydration.

However, the onmy-event prop on <my-element> in <WorksWithForcedRerender> React component properl