Weird behaviour of a multiline prompt

["Nikolay Aleksandrovich Pavlov (ZyX)" <kp-pav@xxxxxxxxx>]

If I have a prompt like

    PS1=$'abc\n'

(i.e. non-empty line above an empty prompt) then it shows up as expected when I replace my prompt with this. But when I try using exactly the same prompt in `pexpect` zsh no longer shows any prompt at all: neither `abc`, nor RPS1. Used environment is

	environ = {
		'LANG': 'en_US.UTF-8',
		'PATH': os.pathsep.join(local_paths),
		'TERM': 'screen-256color',
		'DIR1': os.environ['DIR1'],
		'DIR2': os.environ['DIR2'],
		'XDG_CONFIG_HOME': os.path.abspath(os.path.join('tests', 'shell', 'fish_home')),
		'IPYTHONDIR': os.path.abspath(os.path.join('tests', 'shell', 'ipython_home')),
		'PYTHONPATH': python_paths,
		'POWERLINE_CONFIG_OVERRIDES': os.environ.get('POWERLINE_CONFIG_OVERRIDES', ''),
		'POWERLINE_THEME_OVERRIDES': os.environ.get('POWERLINE_THEME_OVERRIDES', ''),
		'POWERLINE_CONFIG_PATHS': os.path.abspath(os.path.join('powerline', 'config_files')),
		'POWERLINE_COMMAND_ARGS': os.environ.get('POWERLINE_COMMAND_ARGS', ''),
		'POWERLINE_COMMAND': os.environ.get('POWERLINE_COMMAND', ''),
		'LD_LIBRARY_PATH': os.environ.get('LD_LIBRARY_PATH', ''),
	}

(https://github.com/ZyX-I/powerline/blob/bd84cd240ed1ddda8d558079f96d6a47d21206fd/tests/test_shells/run_script.py#L52-L67). Zsh version is 5.2. Any ideas what may be the reason?