Mocking of a function within a function not working in jest with jest.spyOn - jestjs

Question

I'm trying to write a test for a function that downloads an Excel file within my React app.
I understand that I need to mock certain functionality, but it doesn't seem to be working according to everything that I have read online.
A basic mock that works is this:
import FileSaver from 'file-saver'
import { xlsxExport } from './functions'
// other code...
test('saveAs', async () => {
const saveAsSpy = jest.spyOn(FileSaver, 'saveAs')
FileSaver.saveAs('test')
expect(saveAsSpy).toHaveBeenCalledWith('test')
})
The above works: FileSaver.saveAs was successfully mocked. However, I am utilising FileSaver.saveAs within another function that I wish to test and the mocking does not seem to transfer into that. functions.ts and functions.tests.ts below.
functions.ts:
import { Dictionary } from './interfaces'
import * as ExcelJS from 'exceljs'
import FileSaver from 'file-saver'
export function xlsxExport(data: Dictionary<any>[], fileName?: string, tabName?: string) {
const workbook = new ExcelJS.Workbook()
const worksheet = workbook.addWorksheet(tabName || 'export')
// Get columns from first item in data
worksheet.columns = Object.keys(data[0]).map((key: string) => ({ header: key, key: key }))
// Write each item as a row
for (const row of data) {
worksheet.addRow(row)
}
// Download the file
workbook.xlsx.writeBuffer().then(function (buffer) {
const blob = new Blob([buffer], { type: 'applicationi/xlsx' })
FileSaver.saveAs(blob, (fileName || 'excel_export') + '.xlsx')
})
}
functions.tests.ts
import FileSaver from 'file-saver'
import { xlsxExport } from './functions'
// ...other code
test('xlsxExport', async () => {
const saveAsSpy = jest.spyOn(FileSaver, 'saveAs')
xlsxExport(myArrayOfDicts, 'test_download')
expect(saveAsSpy).toHaveBeenCalledWith('something, anything')
})
Error:
TypeError: Cannot read properties of null (reading 'createElement')
at Function.saveAs (C:\dev\pcig-react\node_modules\file-saver\src\FileSaver.js:92:9)
at C:\dev\pcig-react\src\common\functions.ts:221:19
at processTicksAndRejections (node:internal/process/task_queues:95:5)
Node.js v19.3.0
FAIL src/common/functions.test.ts
● Test suite failed to run
Jest worker encountered 4 child process exceptions, exceeding retry limit
at ChildProcessWorker.initialize (node_modules/jest-runner/node_modules/jest-worker/build/workers/ChildProcessWorker.js:185:21)
It is trying to call the non-mocked FileSaver.saveAs (line 221 of my file) within xlsxExport.
How can I get it to call the mocked version?