The defaultThreshold property is not honored as documented :
defaultThreshold Level of messages to be filtered. If there is no matching key in the key/value pairs then this level will be compared against the event's level.
after carefully examining the source code, I found the following code is called:
where level is the event's level, when there's no matching key found contextMap, Result.NEUTRAL is mistakenly returned, instead of against this.defaultThreshold.