Undefined data set name with facet inside concat

[domoritz]

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"values": [{"a": "A", "b": 28}, {"a": "B", "b": 55}]},
  "vconcat": [
    {
      "title": "Title for first row",
      "facet": {"field": "a", "type": "ordinal"},
      "spec": {
        "mark": "bar",
        "encoding": {"y": {"field": "b", "type": "quantitative"}}
      }
    },
    {
      "title": "Title for second row",
      "facet": {"field": "a", "type": "ordinal"},
      "spec": {
        "mark": "bar",
        "encoding": {"y": {"field": "b", "type": "quantitative"}}
      }
    }
  ]
}

Produces this error

Error: Undefined data set name: "scale_concat_1_child_main"

Found in #5432 by @buckle2000.

[domoritz]

Fixed in #7639