After LOGBACK-620, the MDC map is no longer in InheritableThreadLocal.

So when you MDC.put on a thread if you create a new thread from it, the new thread will not receive the value.

I think the LOGBACK-620 was about not copying the map on each thread creation but only when it the map is modified, but the lost of inherited value was unexpected.

If so replacing

final ThreadLocal<Map<String, String>> copyOnThreadLocal = new ThreadLocal<Map<String, String>>();

by 

final ThreadLocal<Map<String, String>> copyOnThreadLocal = new InheritableThreadLocal<Map<String, String>>();

will do the fix