Hello again,
We have tested it with MS Word 2016 and there're two fields when adding the alt text (as other MS Word versions), this is the XML output of the wp:docPr tag:
<wp:docPr descr="$VAR_DESCR_1$" id="826989436" name="" title="$VAR_TITLE_1$"/>
We think the user has added it to the Title field, not the Description one.
Regards.