The issue can be reproduced with custom regional settings and the following mask: dd/MM/yyyy

Workaround: if possible please reset the DateTime region settings

Use the following code to setup RadMaskedEditBox. When you start typing you will notice that the ValueChanged event is fired but the ValueChanging is not: 

            this.radMaskedEditBox1.MaskType = MaskType.Standard;
            this.radMaskedEditBox1.Mask = "AAAA";

            this.radMaskedEditBox1.ValueChanged += radMaskedEditBox1_ValueChanged;
            this.radMaskedEditBox1.ValueChanging += radMaskedEditBox1_ValueChanging;

        private void radMaskedEditBox1_ValueChanging(object sender, CancelEventArgs e)
        {
        }

        private void radMaskedEditBox1_ValueChanged(object sender, EventArgs e)
        {
        }

You need to press Tab twice to move to the next control when in popup editor.

Use attached to reproduce. 

This behavior can be observed in the Office2010 theme. In the following image, the button's width in RadCalculatorDropDown and RadSpinEditor is less by 1 px compared to the buttons in the other editor controls.