Can logback.xml reference a property in a provider declaration?

Question

I was reading the following section in the logback manual: https://logback.qos.ch/manual/configuration.html#variableSubstitution

I'm using logback v2.9.1 and v6.1 of logstash-logback-encoder.

I have a situation where I'm defining an encoder with a "providers" element, and in that I'm setting the "stackTrace" element, and the "throwableConverter". I set the "rootCauseFirst" property. Right now I'm setting it to a static value, but I'm experimenting with setting it from a property.

In my test case, I clearly see the difference between setting a static value of "true" vs. "false". However, if I define a "ROOT_CAUSE_FIRST" variable before the appender definition, with a value of either "true" or "false", and reference "${ROOT_CAUSE_FIRST}" in the "rootCauseFirst" setting, the result is that it doesn't log the exception at all. I don't see it in the log.

The following is an elided copy of my logback.xml:

<configuration scan="true" scanPeriod="5 seconds" debug="true">
        <variable name="ROOT_CAUSE_FIRST" value="false"/>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
            <providers>
                ...
                <stackTrace>
                    <fieldName>exTrace</fieldName>
                    <throwableConverter class="net.logstash.logback.stacktrace.ShortenedThrowableConverter">
                        <maxDepthPerThrowable>10</maxDepthPerThrowable>
                        <!-- <rootCauseFirst>${ROOT_CAUSE_FIRST}</rootCauseFirst> -->
                        <rootCauseFirst>true</rootCauseFirst>
                        <maxLength>10240</maxLength>
                    </throwableConverter>
                </stackTrace>
                <stackHash>
                    <fieldName>exHash</fieldName>
                </stackHash>
                <sequence>
                    <fieldName>seq</fieldName>
                </sequence>
                <pattern><pattern>{ "format": "nf-v1.0" }</pattern></pattern>  
        </providers>
        </encoder>
    </appender>
    ...
</configuration>

What is shown here is the "static" version, and the line with the variable ref is commented out. As I said earlier, this appears to work fine, but if I reverse the comments, it simply doesn't log the exception at all.