I got the same problem in Android Webview, I can't explain what is the source of the bug, but it appear if I manually set the fontSize of the hiddenTextarea it's finally work.

fabricTextbox.on('editing:entered', () => {
    fabricTextbox.hiddenTextarea.style.fontSize = '100px';
});

By default the fontSize is 1px, in my case it start working when I set it at about 50px.

This is really weird particularly because it works when there are characters. Also I notice the input event is not triggered when the textarea is empty with fontSize 1px.