ENTER $H<> sometimes empty?

enter.p6

class A {
    has $.x;

    method WHICH {
       ValueObjAt.new: "A|$!x.WHICH()"
    }
}

# Whose images we collect.
my @G = A.new(x => 1);

my Set $H;
loop {
    $H ∪= @G;
    last if $H<> eqv ENTER $H<>;
    # If this line is commented in (to prevent the infinite loop),
    # it suddenly works as expected, ENTER $H<> is not a Set type object
    # and the loop finishes before $ reaches 20.
    #last if $++ > 20;
    # Prints (Set) infinitely?
    say (ENTER $H<>);
    @G .= map: { A.new(x => (.x + 1) % 13) }
}