I wish did fix the issue. Ensure there is not an implicit style for the text box which is overriding the default expected behavior in this case. IN my case I was using the Cosmopolitan Theme from Microsoft and it had an implicit style for TextBox elements that did not produce the proper behavior. In the resources from that theme if you look at the DefaultTextBoxStyle in the CoreStyles.xaml file, at line 448 you will find the ScrollViewer with a VerticalAlignment set to Center. Adjusting this to top solved my problem.
Appending Date to the end of line in multiple textbox
fixed the issue. Will look into that further If it doesn't have to be a textbox, you can apply some nice formatting using some manipulation of datagridview. Using 2 columns with the formatting and colors changed to match the rest of the controls.