In some cases, using lookups for threadcontext does not output logs.

Hi everyone.

I was checking the operation of 2.16.0.

There, I found a case where no logs were output when I specified lookups 
for ThreadContext.

Here is the code I tested.


[log4j2.xml]
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="off">
   <appenders>
     <Console name="main" target="SYSTEM_OUT">
       <PatternLayout pattern="%m - $${ctx:userid}%n" />
     </Console>
   </appenders>
   <Loggers>
     <Root level="INFO" additivity="false">
       <AppenderRef ref="main"/>
     </Root>
   </Loggers>
</Configuration>


[Java]
import org.apache.log4j.LogManager;
import org.apache.log4j.Logger;
import org.apache.logging.log4j.ThreadContext;

public class Test {
   public static void main(String[] args) {
     Logger logger = LogManager.getLogger(Test.class);

     ThreadContext.put("userid", "${java:version}");
     logger.warn("case1");

     ThreadContext.put("userid", "${java:xxx}");
     logger.warn("case2");

     ThreadContext.put("userid", "${ctx:userid}");
     logger.warn("case3");

     ThreadContext.put("userid", "${ctx:xxx}");
     logger.warn("case4");
   }
}


[stdout]
case1 - Java version 11.0.3
case4 - ${ctx:xxx}


Look at stdout.
There is no output for case2 and case3.

I think this is a bug, but what do you guys think?