Before I touch my Reports spreadsheet, I'm trying to solve this problem on my test sheet with a 'Test' tab in it ... if it helps, my Powershell for this test is;
$xl=New-Object -ComObject Excel.Application
$wb=$xl.WorkBooks.Open('C:\Temp\Excel_Report\Test_Formula.xlsx')
$page1 = 'Test'
$ws = $wb.worksheets | where-object {$_.Name -eq $page1}
$xl.Visible=$false
$formula = "=Unique(B1#)"
$ws.Cells.Item(1,4).Formula = $formula
$wb.Save()
$xl.Quit()