Topichead does not convert properly when ditamap saved as book and hierarchy not maintained

I'm new to DITA and piloting some documents in FrameMaker 12, trying to get the output that I need.

I have created a ditamap with a structure as follows:

Map title

- Concept A

- Topichead

   - Task A

   - Task B

- Concept B

  - Task C

   - Task D

I would like this to ouptut a PDF with TOC and bookmarks as follows:

Map title => book title

- Concept A => Chapter 1

- Topichead => Chapter 2

   - Task A => Chapter 2.1

   - Task B => Chapter 2.2

- Concept B => Chapter 3

  - Task C => Chapter 3.1

  - Task D => Chapter 3.2

To do this I save the ditamap as a "Book 12.0 with fm components (*.book)" and set the Bookmark levels in the PDF Setup as follows:

ChapterTitle

head.0

Title

title.0

.title.1

..title.2

...title.3

However, the output systematically comes out wrong because the Topichead element, which is just a title and is not attached to any topic, is not taken into account as a hierarchical element in the output. It generates an empty fm document containing just the topichead title, output as a ChapterTitle or Title depending on whether I set its toc attribute to yes. But the TOC is all wrong - instead of taking Task A and Task B as children of Chapter 2 it puts them all at level 1 so I get:

- Concept A => Chapter 1

- Topichead => ignored

   - Task A => Chapter 2

   - Task B => Chapter 3

- Concept B => Chapter 4

  - Task C => Chapter 4.1

  - Task D => Chapter 4.2

If I output using the bog-standard DITA-OT instead of fancy and expensive FrameMaker, everything comes out correctly.

Is there any way to correct this?

Thanks,

Archaeopteryx