in log4j-ecs-layout/src/main/java/co/elastic/logging/log4j/MdcAccess.java [73:79]
public Map<String, ?> getMDC(LoggingEvent event) {
if (Thread.currentThread().getName().equals(event.getThreadName())) {
return MDC.getContext();
}
// can't access MDC in async appenders
return Collections.emptyMap();
}