Hello,
If you display the prompt characters, do you see two prompt dots right after the ","? If so, the MaskedTextBox control is meant to match text values against a text mask. The control doesn't really know if the mask is for a number, email address, or any other data. When a number like 10.00 is ToString()'ed, it just converts to text "10". You could possibly work around this by making a value converter that when converting the Double to a String will use the "N2" format.
If you are trying to edit a Double value, the better way to do it would likely be to use our DoubleEditBox control with an "N2" Format. That will bind directly to a Double value, the control is made for editing Doubles, and it will show the decimals with that format set.