My guess at Adobe's logic here is the idea of a dropdown is to hide the content unless the user specifically wants to see it. Not all will. In the minitoc, they might want to see the heading but not necessarily the dropdown content. Effectively the point of the dropdown would be lost.
Please follow this link to report bugs or request new features. https://tracker.adobe.com. Post the link to your bug report / feature request in this thread and others can vote for it. The more people who do so, the higher it gets prioritised.
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.