How to reproduce:
this.radMaskedEditBox1.Font = new System.Drawing.Font("Segoe UI", 20F);
this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard;
this.radMaskedEditBox1.Mask = "#####-######";
this.radMaskedEditBox1.Value = "47997006886";
Workaround:
this.radMaskedEditBox1.Font = new System.Drawing.Font("Segoe UI", 20F);
this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard;
this.radMaskedEditBox1.Mask = "#####-######";
this.radMaskedEditBox1.Text = "47997006886";