Faced a probable bug (at least, did not find any mention about such issue at manual/google/other bugs in category).

Brief logging configuration description:
I have a property defined in my logback configuration.
There is a property with log file location defined:
<property name="LOG_HOME" value="/tmp/aicore" />
Also, I have ConsoleAppender and two RollingFileAppender appenders, attached to root logger. RollingFileAppender configuration contains LOG_HOME variable in path and substitution is done without any issues, log file is created and updated.
But when I add SiftingAppender (log files are created on per-threadId basis), that contains FileAppender in it's sift section, variable value is not substituted.
Here is the logback log fragment, generated using OnConsoleStatusListener:

22:11:27,396 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - About to instantiate appender of type [ch.qos.logback.core.FileAppender]
22:11:27,396 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - Naming appender as [FILE-server]
22:11:27,398 |-INFO in ch.qos.logback.core.joran.action.NestedComplexPropertyIA - Assuming default type [ch.qos.logback.classic.encoder.PatternLayoutEncoder] for [encoder] property
22:11:27,400 |-INFO in ch.qos.logback.core.FileAppender[FILE-server] - File property is set to [LOG_HOME_IS_UNDEFINED/thread-server.log]

Note the "LOG_HOME_IS_UNDEFINED/thread-server.log" value. As a result, no any sift log file is created in expected location. When I substitute ${LOG_HOME} value manually, everything works well.

Below is my SiftingAppender config:

<appender name="SIFT" class="ch.qos.logback.classic.sift.SiftingAppender">
<discriminator class="ch.qos.logback.classic.sift.MDCBasedDiscriminator">
<key>threadId</key>
<defaultValue>unclassified</defaultValue>
</discriminator>
<sift>
<appender name="FILE-${threadId}" class="ch.qos.logback.core.FileAppender">
<file>${LOG_HOME}/thread-${threadId}.log</file>
<append>false</append>
<encoder>
<pattern>%d [%thread] %level %mdc %logger

- %msg%n</pattern>
</encoder>
</appender>
</sift>
</appender>

Sorry if bug report is not formed perfectly, I have not much expertise yet