To reproduce:

Place a MaskedTextBox on a Form
Set 'MaskType' to 'Numeric' (and in my case I have Mask set to 'D' to allow integer values only)
Run the application / open the form - by default the masked textbox has a value of '0'
Elsewhere (i.e. notepad) enter and copy a text like 'CT_1234' into the clipboard (please note the starting non-numeric string characters)
Go back to the application / form and click into the masked text box
Select the pre-existing value (by default  the '0' mentioned above or any other value i.e. '11111' .. and by select I mean via mouse or [Shift]+[Arrow Key Left/Right])
Press [Ctrl+V]
>> Nothing happens
.. what should happen though is that the selected text is overridden with '1234'
However, if you have no text selected in the masked text box and place the cursor in it i.e. at the very beginning or end of the existing value, pasting works as expected.. the value '1234' is added (and correctly 'dropping the 'CT_' in front of it).