The bug is reported to NPOI:
https://github.com/nissl-lab/npoi/issues/187

The buggy code of NPOI is as follows:

private void BeginRow(int rownum, SXSSFRow row1){
...
WriteAsBytes(OutputStream, " customHeight=\"true\"  ht=\"" + row.HeightInPoints + "\"");...
}

The fixed code of NPOI is as follows:

The buggy code of NPOI is as follows:

private void BeginRow(int rownum, SXSSFRow row1){
...
WriteAsBytes(OutputStream, " customHeight=\"1\"  ht=\"" + row.HeightInPoints + "\"");...
}

Still, I find that POI still sets the attribute to true:

beginRow(int,SXSSFRow)@SheetDataWriter
void beginRow(int rownum, SXSSFRow row) throws IOException {
        _out.write("<row");
        writeAttribute("r", Integer.toString(rownum + 1));
        if (row.hasCustomHeight()) {
            writeAttribute("customHeight", "true");
            writeAttribute("ht", Float.toString(row.getHeightInPoints()));
        }...
}