[perl #131763] Merged output of Proc::Async does not work (whenever $proc { … })

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# New Ticket Created by  Aleks-Daniel Jakimenko-Aleksejev 
# Please include the string:  [perl #131763]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=131763 >


Code:

sub get-output() {
    my $proc = Proc::Async.new: ‘echo’, ‘test’;
    react {
        #whenever $proc.stdout { } # ←↙ these work
        #whenever $proc.stderr { } #
        whenever $proc { }         # ← this doesn't
        whenever $proc.start { }
    }
    say $++;
}

loop { get-output }




The result depends on your luck. Sometimes it gets stuck on the second 
execution, sometimes it manages to do it over 20 times and then gets stuck.

According to this it is supposed to work: 
https://irclog.perlgeek.de/perl6-dev/2017-07-12#i_14862898

I don't know if there's any trap I'm getting into, or if it's just a bug. 
Anyway, I hope a ticket won't hurt.