Why does Excel automatically convert the letters "I" in the middle of the text to uppercase?

Question

Macros

The translation is made from numbers to words using a macro in Excel itself.
For me (masOS) everything displays correctly, for the customer (Windows) all the letters "I" in the line become capitalized.
The option with the addition of LOWER (=LOWER(RubLiterally(J23;0;1;0))) did not help, then the translation does not occur at all.
Here is the macro code itself (all Cyrillic in the name of the structures, the function name has been replaced with Latin):

Function РубПропись(Сумма As Double, Optional Без_копеек As Boolean = False, _
 Optional КопПрописью As Boolean = False, Optional начинитьПрописной As Boolean = True) As String
'Функция для написания суммы прописью
 Dim ed, des, sot, ten, razr, dec
 Dim i As Integer, str As String, s As String
 Dim intPart As String, frPart As String
 Dim mlnEnd, tscEnd, razrEnd, rub, cop
 dec = Array("", "одна ", "две ", "три ", "четыре ", "пять ", "шесть ", "семь ", "восемь ", "девять ")
 ed = Array("", "один ", "два ", "три ", "четыре ", "пять ", "шесть ", "семь ", "восемь ", "девять ")
 ten = Array("десять ", "одиннадцать ", "двенадцать ", "тринадцать ", "четырнадцать ", "пятнадцать ", "шестнадцать ", "семнадцать ", "восемнадцать ", "девятнадцать ")
 des = Array("", "", "двадцать ", "тридцать ", "сорок ", "пятьдесят ", "шестьдесят ", "семьдесят ", "восемьдесят ", "девяносто ")
 sot = Array("", "сто ", "двести ", "триста ", "четыреста ", "пятьсот ", "шестьсот ", "семьсот ", "восемьсот ", "девятьсот ")
 razr = Array("", "тысяч", "миллион", "миллиард")
 mlnEnd = Array("ов ", " ", "а ", "а ", "а ", "ов ", "ов ", "ов ", "ов ", "ов ")
 tscEnd = Array(" ", "а ", "и ", "и ", "и ", " ", " ", " ", " ", " ")
 razrEnd = Array(mlnEnd, mlnEnd, tscEnd, "")
 rub = Array("рублей", "рубль", "рубля", "рубля", "рубля", "рублей", "рублей", "рублей", "рублей", "рублей")
 cop = Array("копеек", "копейка", "копейки", "копейки", "копейки", "копеек", "копеек", "копеек", "копеек", "копеек")
 If Сумма >= 1000000000000# Or Сумма < 0 Then РубПропись = CVErr(xlErrValue): Exit Function
 '&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
 If Round(Сумма, 2) >= 1 Then
  intPart = Left$(Format(Сумма, "000000000000.00"), 12)
  For i = 0 To 3
   s = Mid$(intPart, i * 3 + 1, 3)
   If s <> "000" Then
    str = str & sot(CInt(Left$(s, 1)))
    If Mid$(s, 2, 1) = "1" Then
     str = str & ten(CInt(Right$(s, 1)))
    Else
     str = str & des(CInt(Mid$(s, 2, 1))) & IIf(i = 2, dec(CInt(Right$(s, 1))), ed(CInt(Right$(s, 1))))
    End If
    On Error Resume Next
    str = str & IIf(Mid$(s, 2, 1) = "1", razr(3 - i) & razrEnd(i)(0), _
     razr(3 - i) & razrEnd(i)(CInt(Right$(s, 1))))
    On Error GoTo 0
   End If
  Next i
  str = str & IIf(Mid$(s, 2, 1) = "1", rub(0), rub(CInt(Right$(s, 1))))
 End If
 РубПропись = str
 ''''''''''''''''''
 If Без_копеек = False Then
  frPart = Right$(Format(Сумма, "0.00"), 2)
  If frPart = "00" Then
   frPart = ""
  Else
   If КопПрописью Then
    frPart = IIf(Left$(frPart, 1) = "1", ten(CInt(Right$(frPart, 1))) & cop(0), _
     des(CInt(Left$(frPart, 1))) & dec(CInt(Right$(frPart, 1))) & cop(CInt(Right$(frPart, 1))))
   Else
    frPart = IIf(Left$(frPart, 1) = "1", frPart & " " & cop(0), frPart & " " & cop(CInt(Right$(frPart, 1))))
   End If
  End If
  РубПропись = str & " " & frPart
 End If
 ''''''''''''''''''
' РубПропись = str & frPart
 If начинитьПрописной Then Mid$(РубПропись, 1, 1) = UCase(Mid$(РубПропись, 1, 1))
' If начинитьПрописной Then РубПропись = UCase(Left(РубПропись, 1)) & Mid(РубПропись, 2)
End Function