I have a job triggered by CI Event (JMS Messaging provider). The job has a declared parameter CI_MESSAGE of type Multi-line String Parameter, according the config.xml of the job, the parameter is

       <hudson.model.TextParameterDefinition>
          <name>CI_MESSAGE</name>
          <description></description>
          <defaultValue></defaultValue>
        </hudson.model.TextParameterDefinition>
 

When the job is triggered by this plugin, it very ofter puts the content of this parameter as a String, according the build.xml as:

        <hudson.model.StringParameterValue>
          <name>CI_MESSAGE</name>
          <value>...content...</value>
        </hudson.model.StringParameterValue>

although the definition in the same build.xml still says:

            <hudson.model.TextParameterDefinition>
              <name>CI_MESSAGE</name>
              <description></description>
              <defaultValue></defaultValue>
            </hudson.model.TextParameterDefinition>
          </parameterDefinitions>

Very rare the parameter is correctly stored in build.xml as:

        <hudson.model.TextParameterValue>
          <name>CI_MESSAGE</name>
          <description></description>
          <value>...content...</value>
        </hudson.model.TextParameterValue>

and the definition is the same as previous.

UI is confused and String parameter has a far different rendering than Text parameter.

It is related to the other weird state - when it put the content of the parameter as String, it separate each key=value pair to separate line, when it put the content of the parameter as Text, it squash all pairs to the one line as:
Content from hudson.model.StringParameterValue:

<value>build=aaa
param2=128
url=foo</value>

Equal content from hudson.model.TextParameterValue:

<value>build=aaaparam2=128url=foo</value>

I don't know what is the source of CI_MESSAGE content, but it doesn't matter - the content type should be always TextParameter, not the other one (StringParameter).