<Label><TextBlock>Lorem<LineBreak/>ipsum</TextBlock></Label>
You need to use TextBlock because TextBlock accepts as children a collection of Inline objects. So you are giving the TextBlock element three Inline items: Run Text=”Lorem”, LineBreak, and Run Text=”ipsum”.
You can’t do the following:
<Label>Lorem<LineBreak/>ipsum</Label>`
because a label accepts one content child element.
Also, not sure exactly what your use case is but notice I placed a TextBlock inside your Label element. Is it repetitive? Not really, depending on your need. Here’s a good article on the differences between the two elements: Difference between Label and TextBlock