Convert file always works fine here. I suspect some users will not notice this, and the only detrimental thing I think is that kerning and proper opentype features are not enables in compatibility mode. 

On Sunday, 17 September 2017 02:04:28 UTC+8, John MacFarlane wrote:
I think this is harmless.  We aim to produce a document that
can be opened with as many versions of Word as possible.
You can convert it with File -> Convert File.

Is there a problem with producing a docx in compatibility
mode?

+++ tolot27 [Sep 16 17 09:03 ]:
>   Hi!
>   I'm using the the nightly build from pandoc and also tried pandoc
>   1.19.2.1 and getting the same docx output in compatibility mode if I
>   just convert a simple document with the default reference.docx:
>   echo "# Test" | pandoc -o test.docx
>   What is the cause for this compatibility mode?
>   I use it with Windows 10 and Microsoft Office 365 German.
>   --
>   Regards,
>   Mathias
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