Vertical position of wrapping label text not consistent - how to keep the position the same?

There's a problem with the vertical position of a label text in my specific case.
The vertical position of a wrapping label text is changing when the text becomes too long to fit in the given layout.

When the text is only two lines and does fit in the available space everything is fine:
See attached file "LayoutProblem_twolines"

As soon as the text becomes too long for the available space and therefore wraps into a (not visible) third line, its position is moved several pixels downwards:
See attached file "LayoutProblem_threelines"

Any ideas on why the position changes when the text becomes too long? And is there maybe a possibility to keep the position of the text the same although it doesn't fit in the available space? Any ideas are highly appreciated.

Here's the corresponding xaml for the concerned elements:
See attached file "LayoutProblem_xaml"