The issue isn’t in the image itself – a background image only goes as big as the content container is allowing.
And looks like the BB content container which holds the background image isn’t tall enough.
Any chance you can check with their support team first and see if there is a setting you are missing somewhere?
Or if that BB content can be added as a shortcode, then you can use the header element to add it (along with the merge), and try setting it to full screen:
https://docs.generatepress.com/article/header-element-overview/#full-screen