Markers.aggregate behave in unexpected way

[agdula]

Describe the bug
making aggregate of aggregates creates Markers that contain less values than expected

        Marker dir = append("directory", "Some Directory");
        log.info(dir, "processing dir");
        //> processing dir | directory=Some Directory
        
        Marker file = aggregate(dir, append("file", "Some Document"));
        log.info(file, "processing file");
        //> processing file | directory=Some Directory, file=Some Document
        
        Marker line  = aggregate(file, append("line", "42")); // < ------------------------------ PROBLEM
        log.info(line, "processing line");
        //> processing line | line=42
        // expected: 
        //> processing line | directory=Some Directory, file=Some Document, line=42

To Reproduce
Steps to reproduce the behavior:

1. clone git@github.com:agdula/logstash-markers-issue.git
2. run https://github.com/agdula/logstash-markers-issue/blob/master/src/test/java/com/github/agdula/logback/issue/FileProcessorTest.java
3. or run https://github.com/agdula/logstash-markers-issue/blob/master/src/main/java/com/github/agdula/logback/issue/Main.java

Expected behavior
Aggregate of many agggregates should contain all of aggregated markers

1 run. https://github.com/agdula/logstash-markers-issue/blob/master/src/test/java/com/github/agdula/logback/issue/FileProcessorFixedTest.java
2 or run run https://github.com/agdula/logstash-markers-issue/blob/master/src/main/java/com/github/agdula/logback/issue/Main.java

Additional context
https://github.com/agdula/logstash-markers-issue