Clear a GUI.TextArea in iOS build

I'm working on a unity app which is built for iOS. It has text area. When the user type and submit the data I want to clear the data in the text area.

The text area is defined like this

questionStr = GUI.TextArea(new Rect(Screen.width-Screen.width * 3/4, Screen.height-Screen.height*3/4, Screen.width/2, Screen.height/4), questionStr, 500);

and I clear it like questionStr = ""; when the submit button is clicked.

When I play this in Unity, it get cleared and it work as expected.

But when I run this on the iPad as a iOS build, the text area doesn't get cleared after submit.

I'm not clear why it doesn't work in the iPad.

Please tell me how I can clear that text in the iOS build.

Thanks