If you call the GetValueFromArgs method, it will allow the grid to properly track the dependent cells.
Public Function Evaluate_Str(ByVal args As String) As String
Dim c As Integer
Dim r As Integer
Try
ReturnGridRefFromString(args, c, r)
Dim cellModel As GridFormulaCellModel
cellModel = Me.grd.CellModels("FormulaCell")
cellModel.Engine.GetValueFromArg(args)
Return grd(r, c).FormattedText
Catch ex As System.Exception
Return "!! ERROR !!"
End Try
End Function