Conditional opacity does not respect default opacity

[kanitw]

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "A scatterplot showing horsepower and miles per gallons for various cars.",
  "data": {"url": "data/cars.json"},
  "mark": "point",
  "encoding": {
    "x": {"field": "Horsepower", "type": "quantitative"},
    "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
    "opacity": {
      "condition": [{"test": "datum.Horsepower > 100", "value": 0.25}]
    }
  },
  "config": {"point": {"opacity": 0.5}}
}

The left half should get opacity = 0.5, not 1.