`pattern` ignored in top-level string schemas

[skonves]

Given the following schema:

"schemas": {
  "objectId": {
    "type": "string",
    "pattern": "^[0-9a-fA-F]{24}$",
    "description": "MongoDB ObjectId string (24-character hex)"
  },
  "someType": {
    "allOf": [
      { "$ref": "#/components/schemas/objectId" }
    ],
    "description": "REDACTED",
    "nullable": true
  }
}

We get the error

warning openapi-3/invalid-schema: Property pattern is not allowed.
6198        “pattern”: “^[0-9a-fA-F]{24}$“,