The problem seems to be here: https://github.com/xwiki-contrib/confluence/blob/be4c559a8d9b7078471a8a5217114350637ef17c/confluence-syntax-xhtml/src/main/java/org/xwiki/contrib/confluence/parser/xhtml/internal/wikimodel/TableCellTagHandler.java#L38 - we're not only creating an inner group syntax, but that group syntax also inherits the parameters of the table cell. However, what I don't understand is why the style is not duplicated, too. In any case, the proper fix is most likely to just remove that group syntax as it isn't needed anymore - the XWiki syntax renderer should add it when necessary.