Why is Log4j2 JsonLayout + KeyValuePair printing empty logEvent messages

问题

TL;DR If I'm using JsonLayout with a nested KeyValuePair in my log4j2.xml config the resulting log messages are empty. Any ideas why?

Long Story

I'm using Tomcat 8.5.43 with the following logging related JARs:

jackson-annotations-2.9.7.jar
jackson-core-2.9.7.jar
jackson-databind-2.9.7.jar
slf4j-api-1.7.25.jar
jcl-over-slf4j-1.7.25.jar
log4j-api-2.12.0.jar
log4j-core-2.12.0.jar
log4j-jul-2.12.0.jar
log4j-slf4j-impl-2.12.0.jar
tomcat-extras-juli-8.5.2.jar (as tomcat-juli.jar in /bin)
tomcat-extras-juli-adapters-8.5.2.jar
log4j-web-2.12.0.jar (in the webapp)

and the log4j2.xml config:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <Appenders>
  <Console name="consoleappender" target="SYSTEM_OUT">
    <JsonLayout compact="true" eventEol="true" stacktraceAsString="true" locationInfo="true">
      <KeyValuePair key="foo" value="bar"/>
    </JsonLayout>
  </Console>
  <Loggers>
    <Root level="info">
      <AppenderRef ref="consoleappender" />
    </Root>
  </Loggers>
</Configuration>

In the setenv.sh I set the classpath to:

CLASSPATH="$JAVA_HOME/lib/tools.jar\
:$CATALINA_HOME/lib/log4j-api.jar\
:$CATALINA_HOME/lib/log4j-core.jar\
:$CATALINA_HOME/lib/log4j-jul.jar\
:$CATALINA_HOME/lib/jackson-annotations.jar\
:$CATALINA_HOME/lib/jackson-core.jar\
:$CATALINA_HOME/lib/jackson-databind.jar"

And run Tomcat with the additional JVM args:

-Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager
-Dlog4j.configurationFile=file:log4j2.xml"

So every Tomcat, JULI, Apache commons and Slf4j logging should be redirected to Log4J2 in the end.

Unfortunately all I see is this: {"logEvent":"","foo":"bar"}

Instead of something like this: {"thread":"main","level":"INFO","loggerName":"org.apache.catalina.startup.VersionLoggerListener","message":"Server version: Apache Tomcat/8.5.43","endOfBatch":false,"loggerFqcn":"org.apache.logging.log4j.jul.ApiLogger","threadId":1,"source":{"class":"org.apache.juli.logging.impl.Jdk14Logger","method":"log","file":"Jdk14Logger.java","line":87},"threadPriority":5,"instant":{"epochSecond":1562919500,"nanoOfSecond":701000000}}

If I remove the KeyValuePair everything works fine as expected.

What's the issue here?