This is a consequence of putting the searchbar into the header in the same place as the title. They have to be stacked and in order for them to be the same size the are horizontally expanded to the available space. This makes the title centered on the available space rather than the whole headerbar. A solution to is to not horizontally expand the stack - the headerbar then centers this on itself. However this has the side effect of the searchbar also being contracted to the same size as the title - which may be short. If you make the stack expand when the search bar is showing then you get a visual glitch which the designer may not approve of.

To be honest. I am not sure its worth the effort of trying to fix this. It is barely noticeable except when the window is narrow.

I have pushed a simple fix for the issue but as it changes the appearance of the searchbar it may not be approved.