When we upgraded Monolog to 1.17.2 we found an unwanted behavior change in the Logstash formatter. This formatter uses json_encode to format the log record for Logstash consumption. json_encode requires that "All string data must be UTF-8 encoded." When this is violated it will return false instead of an encoded string. Monolog had changed the behavior of their encoder to raise a RuntimeException when this happens rather than just blindly passing the boolean on to the output layer.
This behavior change is not nice for our environment (or any environment arguably) as even attempting to log non-utf-8 data can now trigger a RuntimeException at the point of logging.