agent sends only one value from active item buffer in the first message

Suppose we are monitoring a log file and fill it as follows:

$ echo -ne 'line 1\nline 2\nline 3\nline 4\nline 5\n' > /tmp/logfile.txt

Zabbix agent then sends the following two messages with BufferSend seconds in between:

• at 12:07:57:

  {
      "clock": 1420711677,
      "data": [
          {
              "clock": 1420711677,
              "host": "Zabbix server",
              "key": "log[/tmp/logfile.txt]",
              "lastlogsize": 7,
              "ns": 49700463,
              "value": "line 1"
          }
      ],
      "ns": 49771574,
      "request": "agent data"
  }

• at 12:08:02:

  {
      "clock": 1420711682,
      "data": [
          {
              "clock": 1420711677,
              "host": "Zabbix server",
              "key": "log[/tmp/logfile.txt]",
              "lastlogsize": 14,
              "ns": 50627725,
              "value": "line 2"
          },
          {
              "clock": 1420711677,
              "host": "Zabbix server",
              "key": "log[/tmp/logfile.txt]",
              "lastlogsize": 21,
              "ns": 50790296,
              "value": "line 3"
          },
          {
              "clock": 1420711677,
              "host": "Zabbix server",
              "key": "log[/tmp/logfile.txt]",
              "lastlogsize": 28,
              "ns": 50911200,
              "value": "line 4"
          },
          {
              "clock": 1420711677,
              "host": "Zabbix server",
              "key": "log[/tmp/logfile.txt]",
              "lastlogsize": 35,
              "ns": 51042818,
              "value": "line 5"
          }
      ],
      "ns": 53654876,
      "request": "agent data"
  }

It can be seen that the first line is sent in a separate message, while the rest of the lines are sent in the second message. The agent should send all lines in one message.