The log events generated by https://github.com/wikimedia/change-propagation reuse names for structured context data for values with inconsistent types. event seems to be especially overloaded with both object and string data when added to various different log messages. There also seems to be an inconsistent use of message and msg.
There's nothing wrong with this except when we try to store the log events in Elasticsearch as part of our ELK stack for unified log reporting. See T150106 for some of the issues that this causes there. Self-inconsistencies are bad and type collisions across applications are worse.