We haven't yet been able to reproduce it reliably, but it seems very likely that the SLF4JBridgeHandler may get lost when the JUL rootLogger is being garbage collected, because no strong reference to the rootLogger is retained.

https://docs.oracle.com/javase/7/docs/api/java/util/logging/LogManager.html#addLogger(java.util.logging.Logger):

"The application should retain its own reference to the Logger object to avoid it being garbage collected. The LogManager may only retain a weak reference."