When a log event is recorded via Monolog and the structured data contains a message key, Logstash will show the value of that key instead of the actual message. (A number of other fields are also affected, e.g. type, channel.)
Example (compare message and normalized_message).
This is not exactly unexpected behavior (Logstash just merges the structured data array with the rest of the metadata) but still seems weird and might result in information loss (IIRC the normalized message is cut off at some limit while the original message can be longer). Maybe we should use _message instead?