I don't think this is a bug, but a default functionality of logback. It seems that the log entries with ERROR level are prioritized to be logged instantly and not asynchronous.

You can override this behavior by modifying the configuration of your AsyncAppender adding the neverBlock option to true.
 

<appender name="ASYNC" class="ch.qos.logback.classic.AsyncAppender">
	<neverBlock>true</neverBlock>
	<appender-ref ref="CONSOLE" />
</appender>