In fact the red border is shown at the second attempt to commit the cell, but indeed first commit does not draw a red border. The problem comes from the fact that "VisualStates" for the editor presenter control have equal names with System.Windows.Controls.Control visual states (the problem state is the default one which name is "Valid"). For some reason .NET framework changes state of the control to valid (maybe because there is no binding errors) and since our control has "Valid" states it is applied. We will fix this problem by changing the default visual state name to "Normal" and the problem will be solved with the next internal build (Monday 25 Feb).
I've added 1000 Telerik points to your account accordingly.
All the best,
the Telerik team