Why Stream<T> collect method returns different key order?

I have this code:

public enum Continent {ASIA, EUROPE}

public class Country {      
   private String name;
   private Continent region;

    public Country (String na, Continent reg) { 
        this.name = na;
        this.region = reg;
    }
    public String getName () {return name;} 
    public Continent getRegion () {return region;}
    @Override
    public String toString() {
        return "Country [name=" + name + ", region=" + region + "]";
    }
}

and in main class:

public static void main(String[] args) throws IOException {
        List<Country> couList = Arrays.asList(
            new Country ("Japan", Continent.ASIA), 
            new Country ("Sweden", Continent.EUROPE), 
            new Country ("Norway", Continent.EUROPE));
        Map<Continent, List<String>> regionNames = couList
                .stream()
                //.peek(System.out::println)
                .collect(Collectors.groupingBy(Country::getRegion, Collectors.mapping(Country::getName, Collectors.toList())));
        System.out.println(regionNames);
}

If I run this code, I get this output:

{EUROPE=[Sweden, Norway], ASIA=[Japan]}

but if I uncomment the peek function, I get this output:

Country [name=Japan, region=ASIA]
Country [name=Sweden, region=EUROPE]
Country [name=Norway, region=EUROPE]
{ASIA=[Japan], EUROPE=[Sweden, Norway]}

My question is, can anybody tell me, why the keys order differs in the map regionNames when the peek function is in place?