I was able to reproduce the problem and logged it in our internal bug tracker. The problem comes from the fact that the keydown event on mobile devices in some cases cannot be prevented. And the MaskedTextBox does exactly this - prevents the keydown events in order to control the user input. We will investigate the problem further, however cannot provide you with a specific time frame in which the issue will be resolved, as the problem is platform specific and hard to work around.
Thanks in advance for your patience and understanding.
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