We tested Q3 2013 version and Q3 2013 SP and we can assure you there is no change in a disabled MaskedInputControl in Windows8TouchTheme. Furthermore, the disabled MaskedInput has common look in all themes - the border's Opacity is smaller than normal and the ClearButton is not visible.
Can you point us in which theme and when (in which version) this configuration has been different ?
On the other hand, you can edit the style of the MaskedInputControl and change what the Disabled Visual State does to better serve for your requirements.
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