Karaf 4.2.10: custom log4j2 appender produces duplicated log records

Question

After upgrading Karaf from 4.0.3 (pax-logging-log4j2 1.8.4) to 4.2.10 (pax-logging-log4j2 1.11.7) my custom log4j2 plugin (appender that was written by extending AbstractApp