Hi there! Here are the prerequisites for this bug:
1) KendoUITabstrip sample (it doesn't matter, because is reproduced everywhere)
2) Windows phone 8.1 sdk (tested on real device)
3) Inputs on the page (two in my example). The more inputs the larger the stripe.
After I click one of the inputs, native keyboard comes in and white stripe appears over it. Here are the screenshots of the settings view from the tabstrip sample and the bug. There is no stripe when using 8.0 sdk, but there are other issues with it, so I don't want to roll it back.