How does MySQL handles GROUP BY?

I'm on Windows 10, MySQL Workbench. I'm on MySQL 8.0. Here's the dataset. northwind.sql. https://pastebin.com/bMgjXvfT Objective: **Write a query to get the order ID, customer's name, grand total of each order, and the name of the employee who handled each order. See below the expected first four rows.** Output should look like this. This is the database schema diagram. https://brucebauer.info/assets/ITEC3610/Northwind/Northwind-Sample-Database-Diagram.pdf It's northwind.sql database. This is my query.

SELECT 
    o.orderid,
    c.contactname,
    SUM(od.unitprice * od.quantity),
    CONCAT(e.lastname, ' ', e.firstname) AS emp_name
FROM
    orders o
        INNER JOIN
    customers c ON o.customerid = c.customerid
        INNER JOIN
    orderdetails od ON o.orderid = od.OrderID
        INNER JOIN
    employees e ON o.EmployeeID = e.EmployeeID
GROUP BY emp_name
ORDER BY orderid
LIMIT 4;

But it was producing an error. **Error Code: 1055. Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'northwind.o.OrderID' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by** But if I order by orderid alone, it'll work, Why does it work? Should not it be producing the same error as above?