Multiline not processing event until pipeline stops

[espogian]

Hi, with XML files with the following structure:

<?xml version="1.0"?>
<tag id="1234">
  ...
</tag>

And input config:

codec => multiline {
      pattern => "^<\?xml .*\?>"
      negate => true
      what => "previous"
    }

Logstash 5 is not processing the event until the pipeline stops. This also applies when processing multiple files.
To be clear, if I set stdout { rubydebug } as output, I don't see nothing until I stop the pipeline.

I think these could apply:

github.com/driskell/log-courier/issues/38
github.com/logstash-plugins/logstash-codec-multiline/issues/3

And maybe this:

multiline-not-picking-up-events-immediately/46823/9

However, consider that my files have a newline at the end. I've tried to cat several files:

<?xml version="1.0"?>
<tag id="1234">
  ...
</tag>
<tag id="5678">
  ...
</tag>

In that case the last tag is the one which is not being processed until pipeline stops.

Can't figure out how to solve this issue

[espogian]

Little update: I've noticed that using the multiline filter with Logstash 2.4.0 everything works fine.
However, trying the same multiline filter with Logstash 5.5.0 I receive a config error:

reason=>"Couldn't find any filter plugin named 'multiline'. Are you sure this is correct? Trying to load the multiline filter plugin resulted in this error: LoadError

[jychen65]

I encountered exactly the same error.
Does anyone have any idea on how to solve this?