Jetty MDC handler doesn't provide contextPath - jetty

Question

Jetty 9.3.8.v20160314 running on AWS EC2 Linux machine with Java 1.8.0_51 has two web apps under two different dir+context.xml - webapps/app1 and webapps/app2 with an empty webapps/ROOT directory.
I'm trying to get each webapp to have a separate log with contextPath in its name.
Each app has the following webapps/appX.xml context defined -
<Configure class="org.eclipse.jetty.webapp.WebAppContext">
<Set name="contextPath">/appX</Set>
<Set name="war">
<SystemProperty name="jetty.home" default="."/>/webapps/appX
</Set>
</Configure>
Followed Example: Centralized Logging with Logback and installed webapp-logging module, app is using slf4j Logger and resources/logback.xml is -
<configuration>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d{yyyy-MM-dd HH:mm:ss} %t %c{0} [%p] %m%n</pattern>
</encoder>
</appender>
<appender name="SIFT" class="ch.qos.logback.classic.sift.SiftingAppender">
<discriminator>
<key>contextPath</key>
<defaultValue>unknown</defaultValue>
</discriminator>
<sift>
<appender name="FILE-${contextPath}" class="ch.qos.logback.core.rolling.RollingFileAppender">
<file>${jetty.home}/logs/${contextPath}.log</file>
<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
<!-- daily rollover -->
<fileNamePattern>${contextPath}_%d{yyyy-MM-dd}.log</fileNamePattern>
<!-- keep 30 days' worth of history -->
<maxHistory>30</maxHistory>
<totalSizeCap>1GB</totalSizeCap>
</rollingPolicy>
<encoder>
<pattern>%d{yyyy-MM-dd HH:mm:ss} %t %c{0} [%p] %m%n</pattern>
</encoder>
<append>true</append>
</appender>
</sift>
</appender>
<root level="INFO">
<appender-ref ref="STDOUT"/>
<appender-ref ref="SIFT"/>
</root>
</configuration>
But it all goes to unknown.log. Using servername instead works well and creates separate file for each domain being used to access the server.
So it would seem that contextPath isn't properly assigned by ContextLogHandler.
Why is it empty?