After reading the RFC5424 it seems that there is no such "new line message delimiter":

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4.3.1.  Message Length

   The message length is the octet count of the SYSLOG-MSG in the
   SYSLOG-FRAME.  A transport receiver MUST use the message length to delimit a syslog message
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So I think it must be a Fluentd error. This is what caused my confusion:

From in_syslog.rb (https://github.com/athenahealth/fluent-plugin-newsyslog/blob/master/lib/fluent/plugin/in_newsyslog.rb):

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# syslog family add "\n" to each message and this seems only way to split messages in tcp stream
Coolio::TCPServer.new(@bind, @port, SocketUtil::TcpHandler, log, "\n", callback)
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