rowSpan does not seem to work

when using the following data on the sample page (https://catamphetamine.gitlab.io/write-excel-file/)

[
  [
    {
      "value": "Name",
      "fontWeight": "bold",
"rowSpan": 2
    },
    {
       "value": "Date"
      
    },
    {
      "value": "Cost",
      "fontWeight": "bold"
    },
    {
      "value": "Paid",
      "fontWeight": "bold"
    }
  ],
  [
    {
      "value": ""
     
    },
    {
      "value": new Date(),
      "type": Date,
      "format": "mm/dd/yyyy"
    },
    {
      "value": 1800,
      "type": Number,
      "format": "#,##0.00"
    },
    {
      "value": true,
      "type": Boolean
    }
  ],
  [
    {
      "value": "Alice Brown",
      "type": String
    },
    {
      "value": new Date(),
      "type": Date,
      "format": "mm/dd/yyyy"
    },
    {
      "value": 2599.99,
      "type": Number,
      "format": "#,##0.00"
    },
    {
      "value": false,
      "type": Boolean
    }
  ]
]

the columns A1 and A2 should be merged, however, this does not seem to work. Normal span appears to work fine.