Dynamic page title from SC

In a layout page I defined in the Template Data a variable called page_title:

On a content page using this layout, I defined the page_title in the properties:

However the page title is not updated and stays empty.

When using a static value, like below, the page title is properly set:

The server action I use is returning a value btw, without any errors. Why is this not working with dynamic data from the server action?