When expanding a chapter in ToC another chapter also expands - issue

Hello, I am using RoboHelp HTML 11 for developing content sensitive help for a browser application. I am experiencing some trouble when expanding Table of Contents chapters (books) and I am not sure that this is the expected behaviour or if there is a particular setting in RoboHelp for this issue. I encountered the issue in Firefox and IE.
I'll reproduce the steps using examples, to be more clear:

1. Click the help button on a screen. The help window opens on the screen-related topic. The chapter the selected topic is part of is expanded. Let's call this chapter X. All the other chapters are collapsed.

2. Collapse chapter X (important).
3. Expand another chapter - Y. Chapter X also expands.

And here is the issue: No matter what chapter I expand (without selecting a topic from it), the chapter which contains a selected topic also expands. To continue with my example:

4. Select a topic from chapter Y and collapse both chapters.

5. Expand a third chapter - Z. This time, Y also expands, because I previously selected a topic from it.

The desired behaviour should be that no matter how many chapters are expaned in the ToC and no matter which topic is selected, when clicking a collapsed chapter, only that chapter expands.
I am generating my !SSL! folder using the Responsive HTML5 option in RoboHelp, if it helps.

Thank you!