Using as an example of starting the April 30 version 4.1.7.0429 version AjaxControlToolkitSampleSite.zip shifting the focus on the textbox the page is not erased the mask, the result is visible in the attached figure.
Comments: Confirmed this is an issue in 4.1.7.429 using IE10 and FF. lospalmos comment about using tab vs. enter is true as well. Pressing tab leaves the mask in place, but pressing enter removes the mask. This creates lots of problems for validation that relies on the mask being removed.
Comments: Confirmed this is an issue in 4.1.7.429 using IE10 and FF. lospalmos comment about using tab vs. enter is true as well. Pressing tab leaves the mask in place, but pressing enter removes the mask. This creates lots of problems for validation that relies on the mask being removed.