Incorrect keys for predicted buckets in Moving Average Aggregation

[colings86]

My query:

GET weatherdata/_search
{
  "size": 0,
  "query": {
    "bool": {
      "must": [
        {
          "term": {
            "element": {
              "value": "tmax"
            }
          }
        }
      ]
    }
  },
  "aggs": {
    "maxTMax": {
      "max": {
        "field": "value",
        "script": "_value / 10",
        "format": "###.##"
      }
    },
    "maxAvgTMaxMonth": {
      "max_bucket": {
        "buckets_path": "tMaxHisto>avgTMax",
        "format": "###.##"
      }
    },
    "minAvgTMaxMonth": {
      "min_bucket": {
        "buckets_path": "tMaxHisto>avgTMax",
        "format": "###.##"
      }
    },
    "tMaxHisto": {
      "date_histogram": {
        "field": "date",
        "interval": "quarter"
      },
      "aggs": {
        "avgTMax": {
          "avg": {
            "field": "value",
            "script": "_value / 10",
            "format": "###.##"
          }
        },
        "movavg": {
          "moving_avg": {
            "buckets_path": "avgTMax",
            "model": "holt",
            "window": 12,
            "gap_policy": "skip",
            "predict": 12,
            "settings": {
              "alpha": 0.8
            }
          }
        }
      }
    }
  }
}

Response snippet (the first three bucket are existing buckets, i.e. not predictions. The following 12 buckets are predictions but the bucket keys seem to start at 1986 and decrease by 20 years each time):

            {
               "key_as_string": "2005-07-01T00:00:00.000Z",
               "key": 1120176000000,
               "doc_count": 11,
               "avgTMax": {
                  "value": 28.8,
                  "value_as_string": "28.8"
               },
               "movavg": {
                  "value": 29.15195798473475
               }
            },
            {
               "key_as_string": "2005-10-01T00:00:00.000Z",
               "key": 1128124800000,
               "doc_count": 11,
               "avgTMax": {
                  "value": 29.172727272727272,
                  "value_as_string": "29.17"
               },
               "movavg": {
                  "value": 29.024912216300212
               }
            },
            {
               "key_as_string": "2006-01-01T00:00:00.000Z",
               "key": 1136073600000,
               "doc_count": 4,
               "avgTMax": {
                  "value": 29.525,
                  "value_as_string": "29.52"
               },
               "movavg": {
                  "value": 29.34023027099398
               }
            },
            {
               "key_as_string": "1986-01-01T00:00:00.000Z",
               "key": 504921600000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.34023027099398,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1966-01-01T00:00:00.000Z",
               "key": -126230400000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.28650429104976,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1946-01-01T00:00:00.000Z",
               "key": -757382400000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.232778311105537,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1926-01-01T00:00:00.000Z",
               "key": -1388534400000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.179052331161316,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1906-01-01T00:00:00.000Z",
               "key": -2019686400000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.125326351217094,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1885-12-31T00:00:00.000Z",
               "key": -2650838400000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.071600371272872,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1865-12-31T00:00:00.000Z",
               "key": -3281990400000,
               "doc_count": 0,
               "movavg": {
                  "value": 29.01787439132865,
                  "value_as_string": "1970-01-01T00:00:00.029Z"
               }
            },
            {
               "key_as_string": "1845-12-31T00:00:00.000Z",
               "key": -3913142400000,
               "doc_count": 0,
               "movavg": {
                  "value": 28.96414841138443,
                  "value_as_string": "1970-01-01T00:00:00.028Z"
               }
            },
            {
               "key_as_string": "1825-12-31T00:00:00.000Z",
               "key": -4544294400000,
               "doc_count": 0,
               "movavg": {
                  "value": 28.910422431440207,
                  "value_as_string": "1970-01-01T00:00:00.028Z"
               }
            },
            {
               "key_as_string": "1805-12-31T00:00:00.000Z",
               "key": -5175446400000,
               "doc_count": 0,
               "movavg": {
                  "value": 28.856696451495985,
                  "value_as_string": "1970-01-01T00:00:00.028Z"
               }
            },
            {
               "key_as_string": "1785-12-30T00:00:00.000Z",
               "key": -5806598400000,
               "doc_count": 0,
               "movavg": {
                  "value": 28.802970471551763,
                  "value_as_string": "1970-01-01T00:00:00.028Z"
               }
            },
            {
               "key_as_string": "1765-12-30T00:00:00.000Z",
               "key": -6437750400000,
               "doc_count": 0,
               "movavg": {
                  "value": 28.74924449160754,
                  "value_as_string": "1970-01-01T00:00:00.028Z"
               }
            }

[colings86]

The problem is with [1], as this calculation of the interval does not work if key < 0 since the first bucket will calculate the interval as a negative value which will always be less than the actual interval. Also, I'm not sure this method works for non-fixed intervals like months, where the interval will change depending on what the key is

[1] https://github.com/elastic/elasticsearch/blob/35deb7efea9552528780082143db469afbcd3812/src/main/java/org/elasticsearch/search/aggregations/pipeline/movavg/MovAvgPipelineAggregator.java#L137-145