Already fixed. The issue was caused by the wrong if-clause for checking currentSum in read method in the Read.java file in admin.monthly_summary package.
The original code was as follows:
if (currentSum > Integer.parseInt(commissionRuleFrom[i]) && currentSum < Integer.parseInt(commissionRuleFrom[i]))
It is logically wrong because the currentSum cannot be greater than or less than commissionRuleFrom[i] at the same time.
Other variable should be used to represent the upper bound of the range.
The code was revised as follows:
public String[][] read(DBConnectionInterface conn, String sql_in)
{
	ArrayList<String[]> arrayList = new ArrayList<>();
	String[][] tableData_temp;
	try
	{
		Class.forName("com.mysql.cj.jdbc.Driver");
		ResultSet rs = conn.query(sql_in);

		while (rs.next()) 
		{		        	
			String[] currRecord = new String[showField.length+1];
			String currRaw = "";

			for (int i=0 ; i<showField.length ; i++)
			{
				currRecord[i] = rs.getString(showField[i]);
				if (i == 0)
					currRaw += rs.getString(showField[i]);
				else 
					currRaw += ", " + rs.getString(showField[i]);
			}

			// calculate commission
			int currentSum = Integer.parseInt(rs.getString("SUM(booking_grandtotal)"));
			int currCommission = 0;
			for(int i=0 ; i<commissionRuleFrom.length ; i++)
			{
				if (currentSum > Integer.parseInt(commissionRuleFrom[i]) && currentSum < Integer.parseInt(commissionRuleTo[i]))
				{
					currCommission = (int)(currentSum * ((Double.parseDouble(commissionRuleRate[i]))/100));
				}
			}
			currRecord[showField.length] = String.valueOf(currCommission);

			tableDataRaw.add(currRaw);
			arrayList.add(currRecord);
		} 
		rs.close();

		tableData_temp = new String[arrayList.size()][showField.length];

		for(int i=0 ; i<arrayList.size() ; i++)
		{
			tableData_temp[i] = arrayList.get(i);
		}
	
		return tableData_temp;
	}
	catch (Exception e)
	{
		e.printStackTrace();
		tableData_temp = new String[0][0];
		return tableData_temp;
	}
}