I can verify this is occurring to your project. I was unable to achieve the ‘fill parent’ setting with the label. You can use the 100% setting while the label is in a column and achieve what you are asking for
I made some changes to the project to better explain my problem.
I need to align the margin of the label with the margin of a ListViewer (30px for both). If I set the width to 100%, I cannot adjust the margin in pixels. You cannot use “%” to adjust the width of the ListViewer. That is why I was using “Fill container”.