Thank you for your answer and test project.
I've tried to reproduce the problem with your project, but I can't. I've also tris to solve the problem on my side by simplifying this part of the UI, removing the template selector, the custom styles, etc, but the problem still shows.
The main difference between your project and mine is that, in my case, the grid view is defined as a custom control. After copying the simplified XAML code to the right place, the text alignment is working correctly.
So I think that the culprit is the use of the grid view in a custom control.
If you want, I can create a private ticket and give you access to the source code, but it's huge and written in RemObjects Oxygene.
Tell me if I should give you more information.