EDIT: This issue (of course) relates to the case where the combobox has data selected when rendering.
The clear button of the combobox behaves in a way that I find strange, and which prevents us to get the desired behaviour in our application.
It is easily reproducable with your demo here:
When loading the page there is no clear button, which I think is wrong. Especially since it is not even in the DOM, so it cannot be made visible with styling. When clicking in the field, the clear button appears, and it stays there when changing focus to elsewhere, which seems inconsistent.
In other words, the span of the button (in the red box below) only appears when the combobox has had focus.