It turned out that method org.slf4j.impl.Log4jMDCAdapter#getCopyOfContextMap() may return null however org.slf4j.impl.Log4jMDCAdapter#setContextMap() does not handle this situation. That causes the Slf4j API consumer to eventually break with NPE in this code, depending on implementation / current logging backend provider:

Map<String, String> mcdContextMap = MDC.getCopyOfContextMap();
...
MDC.setContextMap(mcdContextMap);

It is expected that MDC.setContextMap() API treats null map as empty map.