Trouble with numbering in hierarchical book (unstructured FrameMaker)

I have an unstructured with a large number of chapter files and would like to group several of the chapters into a single chapter. The autonumbering for the heading paragraph styles is set up like this:

Heading 1 is H:<$chapnum>
Heading 2 is H:<$chapnum>.<n+1>< =0>
Heading 3 is H:<$chapnum>.<n>.<n+1>< =0>
Heading 4 is H:<$chapnum>.<n>.<n>.<n+1>

I created a hierarchical book that looks like this:

Chapter 1.fm
Chapter 2.fm
Folder1 (Numbering Chapter no. is set to continue from previous chapter)
---> New Chapter.fm (H1 top level; numbering Section no. is set to 0)
---> Group1
------> Chapter 23.fm (H2 top level; numbering Section no. is set to continue from previous section)
------> Chapter 10.fm (H2 top level; numbering Section no. is set to continue from previous section)
------> Chapter 15.fm (H2 top level; numbering Section no. is set to continue from previous section)

and so on.

The autonumbering Chapter 23.fm works as expected:

H2 = 3.1
H3 = 3.1.1
H4 = 3.1.1.1

However, H2 in all subsequent chapters in Folder1/Group1 is 3.1 instead of 3.2, 3.3, 3.4, etc.

What am I missing?