According to: https://msdn.microsoft.com/en-us/library/system.diagnostics.process.beginerrorreadline(v=vs.110).aspx
It follows the same logic as: https://msdn.microsoft.com/en-us/library/system.io.streamreader.readline(v=vs.110).aspx
Which makes the statement:
A line is defined as a sequence of characters followed by a line feed ("\n"), a carriage return ("\r"), or a carriage return immediately followed by a line feed ("\r\n").
Mono's BeginErrorReadLine only treats \n as the line deliminator.