BUG when sorting (realtime) topN results by postAggregation vs aggregation

[max-schmidt54321]

I'm running a simple query to get the trend of pageviews on recent data (middle manager):

{
   "queryType":"topN",
   "dataSource":"pageviews",
   "dimension":"onlineId",
   "metric": {
    "type": "numeric",
    "metric": "score"
	},
   "granularity": {"type": "duration", "duration": 1900000, "origin": "2018-09-25T09:12:00.000Z"},
   "threshold": 60,
   "intervals":[
      "2018-09-25T09:12:00.000Z/PT30M"
   ],
   "aggregations":[
      {
         "type":"filtered",
         "filter":{
            "type":"interval",
            "dimension":"__time",
            "intervals":[
               "2018-09-25T09:12:00.000Z/PT15M"
            ]
         },
         "aggregator":{
            "type":"longSum",
            "name":"total_old",
            "fieldName":"count"
         }
      },
      {
         "type":"filtered",
         "filter":{
            "type":"interval",
            "dimension":"__time",
            "intervals":[
               "2018-09-25T09:27:00.000Z/PT15M"
            ]
         },
         "aggregator":{
            "type":"longSum",
            "name":"total_new",
            "fieldName":"count"
         }
      }
   ],
   "postAggregations":[
      {
         "type":"arithmetic",
         "name":"score",
         "fn":"-",
         "fields":[
            {
               "type":"fieldAccess",
               "fieldName":"total_new"
            },
            {
               "type":"fieldAccess",
               "fieldName":"total_old"
            }
         ]
      }
   ]
}

The outcomes vary based on which metric I select for sorting. "score" returns wrong values, while "total_new" returns correct results.

Example Results:

Sorted by postAggregation "metric":"score"

"result": [
            {
                "total_old": 344,
                "onlineId": "10264391",
                "total_new": 1424,
                "score": 1080
            },
            {
                "total_old": 134,
                "onlineId": "6372612",
                "total_new": 606,
                "score": 472
            },
            {
                "total_old": 12,
                "onlineId": "10271038",
                "total_new": 263,
                "score": 251
            },
            {
                "total_old": 53,
                "onlineId": "10261042",
                "total_new": 285,
                "score": 232
            },
	    ...
]

Sorted by aggregation "metric":"total_new"

"result": [
            {
                "total_old": 1250,
                "onlineId": "10264391",
                "total_new": 1424,
                "score": 174
            },
            {
                "total_old": 421,
                "onlineId": "6372612",
                "total_new": 606,
                "score": 185
            },
            {
                "total_old": 408,
                "onlineId": "10271038",
                "total_new": 360,
                "score": -48
            },
            {
                "total_old": 251,
                "onlineId": "10261042",
                "total_new": 285,
                "score": 34
            },
	    ...
]

Notes:

• Druid v0.12.3
• Data is ingested from Kafka
• When I set the threshold high enough, both sortings return the correct results
• When I query data from the historicals (-1day), both sortings return the correct results

[max-schmidt54321]

UPDATE
Same query works for different arithmetic functions (*, +, /). Only subtraction produces wrong results.

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