I managed to find a workaround: if I specify the target cell directly, the new cell will be put into the right place:
{{#xlsxReplace "xl/worksheets/sheet1.xml" "worksheet.sheetData[0].row[1].c[0]"}}
<c t="inlineStr" r="B3"><is><t>Date: 2/9/2018</t></is></c>
{{/xlsxReplace}}