To reproduce, use the following code:
        Dim decimalCol As New GridViewMaskBoxColumn
        decimalCol.FieldName = "DecimalPhone"
        Me.RadGridView1.Columns.Add(decimalCol)
        decimalCol.DataType = GetType(Decimal)
        decimalCol.FormatString = "{0:(###) ###-####}"
        decimalCol.MaskType = MaskType.Standard
        decimalCol.Mask = "(###) ###-####"

Try the following cases:
1. Go to an empty cell and press "1" -> the result is that you see the mask "(1__) ___-_____", however the caret is before it and continuing to type in the editor will replace the already added character
2. Click on an empty cell - the cell is opened for edit, however, the mask is not displayed. Furthermore, when you start typing and you press "1" -> the result is that you see the mask "(1__) ___-_____", however the caret is before it and continuing to type in the editor will replace the already added character