calling .parse in a TOP action causes problems

gistfile1.txt

use Test;

grammar P {  token TOP     {  a  } }
class P::A {  method TOP ($/) {  } }
sub a { P.parse: 'a', :actions(P::A); }
ok P.parse('a', :actions(P::A)) eq 'a', 'call to parse';

grammar G { token TOP { a } }
class G::A {  method TOP($/)  {  a() } }
ok G.parse('a', :actions(G::A)) eq 'a', 'indirect call to parse within a parse';


class G::B {  method TOP($/)  {  P.parse: 'a', :actions(P::A);  } }
ok try { G.parse('a', :actions(G::B)) } eq 'a', 'direct call to parse within a parse';
G.parse('a', :actions(G::B)); # same but without a 'try {}'


=begin END

ok 1 - call to parse
ok 2 - indirect call to parse within a parse
not ok 3 - direct call to parse within a parse
Cannot assign to a readonly variable or a value
  in method parse at src/gen/CORE.setting:10877
  in method TOP at o.p6:13
  in any !reduce at src/stage2/QRegex.nqp:667
  in any !cursor_pass at src/stage2/QRegex.nqp:631
  in regex TOP at o.p6:8
  in method parse at src/gen/CORE.setting:10877
  in block  at o.p6:15


Parsing within a parse causes all sorts of (related?) problems.
Seems related to actions.
Here is my first attempts at reducing the problem to minimal code.
What it does : the Grammar G  TOP action calls a parser in grammar P.
If I use an indirection, doing the P parser call within the function A, there is no problem.