docx4j 8.2.9 is now in Maven Central.
The driver for this release is to ensure the namespace corresponding to the w16sdtdh is declared when docx4j manipulates a docx containing that prefix (which will be the case for documents edited in Word following recent updates).
http://schemas.microsoft.com/office/wor ... dtdatahash is new in v15.0. Word (following recent updates) writes this namespace prefix to @mc:Ignorable in the MDP, for example, "w14 w15 ...