From @moritz

grammar ActionsTestGrammar {
  token TOP {
  ^ .+ $ {*}
  }
}
class TestActions {
  method TOP($/) {
  "a\nb".subst(/\n+/, '', :g);
  make 123;
  }
}

say ActionsTestGrammar.parse("ab\ncd", :action(TestActions.new)).ast;

Prints​:
ab
cd

commenting out the line with the call to .subst makes it print 123 as
expected.

Found by takadonet++

Cheers,
Moritz

From @moritz

works now, needs tests.

@moritz - Status changed from 'new' to 'open'

From @moritz

tested in action-stubs.t

@moritz - Status changed from 'open' to 'resolved'