No. It is not guaranteed that lines will remain intact. They can become intermingled.
From searching based on liori’s answer I found this:
Write requests of {PIPE_BUF} bytes or less shall not be interleaved with data from other processes doing writes on the same pipe. Writes of greater than {PIPE_BUF} bytes may have data interleaved, on arbitrary boundaries, with writes by other processes, whether or not the O_NONBLOCK flag of the file status flags is set.
So lines longer than {PIPE_BUF} bytes are not guaranteed to remain intact.