Yes, and I'm still puzzled. use open should affect the way Test/More.pm opens its todo_output and failure_output filehandles (*Test::More::TESTOUT and *Test::More::TESTERR, respectively); but somehow, open(TESTOUT, ">&STDOUT") does not respect the default stack in ${^OPEN} nor copies the stack on the STDOUT handle. A mistery to me. Anyway, the _real_ solution was already found:

binmode(($_->todo ? $_->todo_output : $_->failure_output), ':utf8')
  for Test::More->builder
[download]

(this one I tested !!) :-)