Nested definition lists in DOCX with pandoc

I have a Markdown file with a nested type of a Definition List. See the example with some terms and nested terms:

# Introduction

This is a Definition List Example:

Term 1

:   Definition 1

    With a paragraph
    
    Term 1a
    
    :   Definition 1a
    
    Term 1b
    
    :   Definition 1b

Term 2

:   Definition 2

Building this with the html5 builder is done via following command:

pandoc -s --toc --toc-depth=3 --number-sections -t html5 -c style/design.css --template=style/template.html Definition_list_test.md -o Definition_list_test.html

and gives an output like following structure:

<h1 data-number="1" id="introduction"><span
class="header-section-number">1</span> Introduction</h1>
<p>This is a Definition List Example:</p>
<dl>
<dt>Term 1</dt>
<dd>
<p>Definition 1</p>
<p>With a paragraph</p>
<dl>
<dt>Term 1a</dt>
<dd>
<p>Definition 1a</p>
</dd>
<dt>Term 1b</dt>
<dd>
<p>Definition 1b</p>
</dd>
</dl>
</dd>
<dt>Term 2</dt>
<dd>
<p>Defintion 2</p>
</dd>
</dl>

Here, the nested type of Term 1a and Term 1b are considered and indented correctly. But using the docx builder, the nested structure is lost:

pandoc -s --reference-doc=style/custom_style.docx Definition_list_test.md -o Definition_list_test.docx

gives an output like following structure.

How can the nested type of Term 1a and Term 1b be considered in docx? I tried to alter the format definitions Definition Term and Definition in the docx template and also tried to add custom styles via [Term 1a]{custom-style="Definition Term 2"}. But, nothing worked so far.