Multiline Java Stack Trace parsing doesn't work

Elastic Stack Beats
2

I've followed Filebeat multiline regexp patterns help which uses the following pattern:

multiline.pattern: ^\d\d\d\d-\d\d-\d\d
multiline.negate: true
multiline.match: after

which I'm testing on: https://play.golang.org/p/IZagant7bJN

Which passes. I still see in the logfiles the following

{
  "@timestamp": "2020-04-14T09:24:07.385Z",
  "@metadata": {
    "beat": "filebeat",
    "type": "_doc",
    "version": "7.6.2"
  },
  "host": {
    "name": "iws-02"
  },
  "agent": {
    "hostname": "iws-02",
    "id": "7c3b9efe-6ddb-4fac-99b9-b47855873fd1",
    "version": "7.6.2",
    "type": "filebeat",
    "ephemeral_id": "402e53ea-fb3e-4e82-b90a-519fe7913122"
  },
  "log": {
    "offset": 25105,
    "file": {
      "path": "/srv/iws/logs/iws-connector-config-service/iws-connector-config-webservice.log"
    }
  },
  "message": "2020-04-14 09:23:57.417 ERROR 1 --- [elastic-1358] s.ConnectorConfigServiceExceptionHandler : Exception handled",
  "input": {
    "type": "log"
  },
  "ecs": {
    "version": "1.4.0"
  }
}

{
  "@timestamp": "2020-04-14T09:24:07.386Z",
  "@metadata": {
    "beat": "filebeat",
    "type": "_doc",
    "version": "7.6.2"
  },
  "agent": {
    "hostname": "iws-02",
    "id": "7c3b9efe-6ddb-4fac-99b9-b47855873fd1",
    "version": "7.6.2",
    "type": "filebeat",
    "ephemeral_id": "402e53ea-fb3e-4e82-b90a-519fe7913122"
  },
  "log": {
    "file": {
      "path": "/srv/iws/logs/iws-connector-config-service/iws-connector-config-webservice.log"
    },
    "offset": 25218
  },
  "message": "com.itembase.iws.connector.config.ConnectorConfigNotFoundException: Requested configuration was not found",
  "input": {
    "type": "log"
  },
  "ecs": {
    "version": "1.4.0"
  },
  "host": {
    "name": "iws-02"
  }
}


{
  "@timestamp": "2020-04-14T09:24:07.386Z",
  "@metadata": {
    "beat": "filebeat",
    "type": "_doc",
    "version": "7.6.2"
  },
  "input": {
    "type": "log"
  },
  "ecs": {
    "version": "1.4.0"
  },
  "host": {
    "name": "iws-02"
  },
  "agent": {
    "hostname": "iws-02",
    "id": "7c3b9efe-6ddb-4fac-99b9-b47855873fd1",
    "version": "7.6.2",
    "type": "filebeat",
    "ephemeral_id": "402e53ea-fb3e-4e82-b90a-519fe7913122"
  },
  "log": {
    "offset": 25324,
    "file": {
      "path": "/srv/iws/logs/iws-connector-config-service/iws-connector-config-webservice.log"
    }
  },
  "message": "\tat com.itembase.iws.connector.config.webservice.connection.ConnectionEntityService.getById(ConnectionEntityService.java:80) ~[classes/:na]"
}

...

So it's still not aggregated in one message but splitted across a lot of events.

I really don't understand what's wrong here.