Why does sheet.js not read formular generated excel data?

Question

So i have a birthday excel which looks like this: (Sorry i haven't found a better method to upload the excel data..)

And i want to read my excel with sheet.js but i can't figure out what i'm missing or doing wrong. So the Age ("Alter" in my Screenshot) is generated by a simple formular which takes the birthdate and subtracts it from the current date.The birthdate is typed in staticly. Just FYI

Now i have the following code:

const path = require("path");
const xlsx = require("xlsx");

const filePath = path.resolve(__dirname, "Mitarbeiter Geburtstagsliste.xlsx");

const workbook = xlsx.readFile(filePath);
const sheetNames = workbook.SheetNames;
//console.log(workbook.Sheets[sheetNames[0]])

const data = xlsx.utils.sheet_to_json(workbook.Sheets[sheetNames[0]]);

data.map(person => {
    console.log(`${person.Name} is ${person.Alter} old!`)
})

And when i console log i get undefined for every param as output, why is that?

undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!
undefined is undefined old!