javax.persistence.criteria.Order.reverse() method javadoc explicitly declares:

public interface Order {
   /**
    * Switch the ordering.
    * @return a new <code>Order</code> instance with the reversed ordering
    */
    Order reverse();

This interface is implemented by two classes:
1. In master branch (5.4.x versions) - by org.hibernate.query.criteria.internal.OrderImpl with this implementation:

...
	public Order reverse() {
		ascending = !ascending;
		return this;
	}
...

It returns this instead of new object

2. In wip/6.0 branch there is another class
org.hibernate.query.sqm.tree.select.SqmSortSpecification that has the same problem:

	@Override
	public JpaOrder reverse() {
		this.sortOrder = this.sortOrder == null ? SortOrder.DESCENDING : sortOrder.reverse();
		return this;
	}

Proposal: respect the contract and return new created object.