The following stack trace copied from above is pretty informative.

Caused by java.lang.NullPointerException
       at org.slf4j.helpers.SubstituteLogger.isDelegateEventAware(SubstituteLogger.java:353)
       at org.slf4j.LoggerFactory.playRecordedEvents(LoggerFactory.java:196)
       at org.slf4j.LoggerFactory.bind(LoggerFactory.java:151)

At the point where LoggerFactory.playRecordedEvents throws a NPE, the is invoked the _delegate variable in SubtituteLogger is supposed to be set. Indeed, the _delegate just a few lines above by the LoggerFactory.fixSubstitutedLoggers method which precedes the call to playRecordedEvents.

I am at a loss on how _delegate variable in SubtituteLogger could be null at that stage.

The code in question was introduced in 1.7.15. Thus, it is pretty new.