My code to find longest string in column doesn't work properly! Need Help!!

Hi everyone,

i had a vba code that finds the longest string in a column and its location contains only "1"s.
It works at the sheet named "datasheet" (first sheet) but at the other sheets like "calc" and "charts" its useless.
What is the problem with this code? Additionally, i was having problem with the activating the sheet, it could be the reason?

Option Explicit
Sub macro1()
   Dim lastRow As Long, elem
   Dim myCount As Long, myRow
   Dim nrCells As Long
   
   Application.ScreenUpdating = False
   With ThisWorkbook.ActiveSheet
      .AutoFilterMode = False
      lastRow = .Cells(Rows.Count, "a").End(xlUp).Row
      .Range("a:a").AutoFilter Field:=1, Criteria1:="<>"
      myCount = 0
      For Each elem In Range("a1:a" & lastRow).SpecialCells(xlCellTypeVisible).Areas
         nrCells = elem.Cells.Count
         If nrCells > myCount Then
            myCount = nrCells
            myRow = elem.Cells(1).Row
         End If
      Next elem
      .AutoFilterMode = False
   End With
   Application.ScreenUpdating = True
   MsgBox "Max number is: " & myCount & " at rows: " & myRow _
         & "-" & myRow + myCount - 1
End Sub

here is a example file of it.

example.xlsx

Thanks in advance