Reporting a potential bug in Vega's joinaggregate transform

df1.csv

df2.csv

joinaggregate_bug.vg.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "signals": [
    {
      "name": "url",
      "bind": {
        "input": "select",
        "options": [
          "https://gist.githubusercontent.com/iaindillingham/91225ed53e42a4cdc3c97f61c5ffe6b9/raw/468159226ee5b34e01e0fbf36f9b4f5d558636db/df1.csv",
          "https://gist.githubusercontent.com/iaindillingham/91225ed53e42a4cdc3c97f61c5ffe6b9/raw/468159226ee5b34e01e0fbf36f9b4f5d558636db/df2.csv"
        ]
      },
      "value": "https://gist.githubusercontent.com/iaindillingham/91225ed53e42a4cdc3c97f61c5ffe6b9/raw/468159226ee5b34e01e0fbf36f9b4f5d558636db/df1.csv"
    }
  ],
  "data": [
    {
      "name": "table",
      "format": {
        "type": "csv"
      },
      "url": {
        "signal": "url"
      },
      "transform": [
        {
          "type": "aggregate",
          "groupby": [
            "dimension"
          ],
          "fields": [
            "metric"
          ],
          "ops": [
            "sum"
          ]
        },
        {
          "type": "joinaggregate",
          "fields": [
            "sum_metric"
          ],
          "ops": [
            "sum"
          ]
        }
      ]
    }
  ]
}