org.apache.wicket.util.tester.WicketTester#assertComponentOnAjaxResponse(java.lang.String) uses org.apache.wicket.util.tester.BaseWicketTester#getComponentFromLastRenderedPage(java.lang.String, true) to get the component and because of the second parameter being "true" it fails to find it.
It should use "false" to be able to find components with outputMarkupPlaceholder = true too.