runtime.onMessageExternal API doesn't work

According to Assessing Your Safari Web Extension’s Browser Compatibility and https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessageExternal, runtime.onMessageExternal API should work in Safari 14. However, the reality is different.

Scenario:

1. having two web extensions; say A (with runtime.id = '123'), and B (with runtime.id = 'xyz').
2. go to the Web Inspector of B and run this:

chrome.runtime.onMessageExternal.addListener((message, sender, sendResponse) => {console.log(${sender.id})})

This should make B wait for an external message.

1. go to the Web Inspector of A and run this:

chrome.runtime.sendMessage('xyz', {message: 'whatever'})

Expect:

• '123' is printed on the Web Inspector of B
• This works perfectly fine on Chrome.

Actual:

• Nothing on the Web Inspector of B
• Got a Promise on the Web Inspector of A -- {result: undefined, status: "resolved"}

Please confirm this is a bug and please fix.