[perl #127036] .flatmap doesn't flatten

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #127036]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=127036 >


It seems .flatmap method does not flatten.

Running the example in the docs produces wrong result:
<ZoffixWin> m: say ((1, 2), <a b>).flatmap(&uc).join('|');
<camelia> rakudo-moar a563e8: OUTPUT«1 2|A B␤»

Also, this:
<LLamaRider> m: say ([1,2],[3,4]).flatmap({$^x}).perl; say 
([1,2],[3,4]).flat.flatmap({$^x}).perl;
<camelia> rakudo-moar a563e8: OUTPUT«($[1, 2], $[3, 4]).Seq␤(1, 2, 3, 4).Seq␤»

Relevant IRC conversation: http://irclog.perlgeek.de/perl6/2015-12-26#i_11781049