After investigating the problem, it turns out that Microsoft introduced a new namespace prefix - w16sdtfl. The current version of Docx4j does not support this namespace, which causes edited templates to produce corrupted report files.
The fix for Jmix 2.x includes an updated version of Docx4j. However, for Jmix 1.x, we cannot upgrade to the latest version because Docx4j uses Jakarta namespaces in its code, which in some cases are incompatible with other dependencies. For full details, you can refer to this issue and the comment with explanation: JAXB-API has not been found on module path or classpath · Issue #4473 · jmix-framework/jmix · GitHub
The only workaround currently is to use another available applications to edit *.docx files, e.g. LibreOffice.