Page header does not have any responsively issue. And the subtitle does not affect it whatsoever. See this page of yours in mobile layout, screenshot.
We have already tested theme’s responsive layout in all devices.
This does not happen on our side. I have recreated the same situation and the title is ok. See screenshot 1 and screenshot 2. It must be something else in your site dictating it. I suggest you disable all third party plugins first and check again. Did you add any custom script recently?
