libreoffice convert docx to odt lost bookmark - pdf

Question

I use the following command converting a docx to odt, one of the bookmarks get lost.
"libreoffice --headless --convert-to odt output.docx --outdir ./"
----- output.docx
------- following bookmarks was created by python-docx
<w:p w14:paraId="5D4F37CC" w14:textId="1FE5A142" w:rsidR="007E1B42" w:rsidRDefault="00356283" w:rsidP="00356283">
<w:pPr>
<w:pStyle w:val="Heading1"/>
</w:pPr>
<w:r>
<w:t>a</w:t>
</w:r>
<w:bookmarkStart w:id="0" w:name="xxbookmark4"/>
<w:bookmarkEnd w:id="0"/>
</w:p>
<w:p w14:paraId="34895FBC" w14:textId="518CEA5B" w:rsidR="007E1B42" w:rsidRDefault="00CF7EC0" w:rsidP="00CF7EC0">
<w:pPr>
<w:pStyle w:val="Heading2"/>
</w:pPr>
<w:r>
<w:t>b</w:t>
</w:r>
<w:bookmarkStart w:id="0" w:name="xxbookmark5"/>
<w:bookmarkEnd w:id="0"/>
</w:p>
...
<w:p>
<w:pPr>
<w:pStyle w:val="Heading1"/>
</w:pPr>
<w:r>
<w:t>Index1</w:t>
</w:r>
<w:bookmarkStart w:id="0" w:name="xxbookmark34"/>
<w:bookmarkEnd w:id="0"/>
</w:p>
<w:p>
<w:r>
<w:t>--a......................................................</w:t>
</w:r>
<w:hyperlink w:anchor="xxbookmark4" w:history="1">
<w:r>
<w:rPr>
<w:rStyle w:val="Hyperlink"/>
</w:rPr>
<w:t>AAA</w:t>
</w:r>
</w:hyperlink>
</w:p>
<w:p>
<w:r>
<w:t>----b................................................</w:t>
</w:r>
<w:hyperlink w:anchor="xxbookmark5" w:history="1">
<w:r>
<w:rPr>
<w:rStyle w:val="Hyperlink"/>
</w:rPr>
<w:t>AAA</w:t>
</w:r>
</w:hyperlink>
</w:p>
<w:p>
<w:r>
<w:t>--Index1.......................................</w:t>
</w:r>
<w:hyperlink w:anchor="xxbookmark34" w:history="1">
<w:r>
<w:rPr>
<w:rStyle w:val="Hyperlink"/>
</w:rPr>
<w:t>AAA</w:t>
</w:r>
</w:hyperlink>
</w:p>
<w:p/>
---- output.odt
<text:h text:style-name="Heading_20_1" text:outline-level="1">
a
<text:bookmark text:name="xxbookmark4"/>
</text:h>
<text:h text:style-name="Heading_20_2" text:outline-level="2">
b
<text:bookmark text:name="xxbookmark5"/>
</text:h>
...
<text:h text:style-name="P1" text:outline-level="1">Index1</text:h>
# issue here: xxbookmark34 is lost.
Index1 was created using python-docx, the other two headings are just edit manually in Word.
Anybody can help, or give some hint.
I compared the xml file in docx and can not find any different between the "Index1" and other two.