The page title is also in the index.html and login.html files.
Mendix does dynamically update those but you may see the original title from these files for a short period of time.
I think the only way to prevent that it to also change the index and login files.
Can you specify where you have changed the title?
If you have changed it here (see image) in the navigation menu, saved it en rerun the project, the title should be changed