Replicated on v5.14.1, MSWin32-x86-multi-thread. Expected behavior is also recovered if the hash is explicitly copied.

use strict;
use warnings;

use threads qw(yield);
use threads::shared;
use Data::Dumper;


sub test { 
    my %h : shared; 
    for (1..3) { 
        threads->new(sub { 
            my($n) = @_;
            lock(%h); 
            $h{$n} = rand(); 
            threads->yield(); 
        }, $_)->join();
    }
    my %i = %h;
    return %i;
} 

my %k = test(); 
print Dumper(\%k);
[download]

or if some games are played with reference and dereference, a la return %{\%h};

Unfortunately, I don't really have anything else useful to add.