As of aroung 16:20 UTC on Jun 13 for some reason logstash started showing complete log entry serialized as JSON in the message field instead of the actual message taken from the log entry.
Example for the same type of the log message:
16:22 https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2018.06.13/cpjobqueue?id=AWP59E_WoOODFPKv9AEI&_g=h@44136fa
16:21 https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2018.06.13/cpjobqueue?id=AWP589O3oOODFPKv8_a7&_g=h@b9a3c59
This happened around the same time for all node services and non of them was deployed around that time, so I'm pretty sure the problem is not in the services themselves.