If you drag in the Navbar set the Expanded option to something other than Always. Then disable Brand and Preview the site in a browser such as Chrome and use the Developer Tools (F12). Then use the toggle device toolbar and select the viewport size to whatever causes the toggle nav button to appear. The toggle button when clicked does not expand the nav. If you go back into BSS4 and toggle on Brand and go back to the preview (refresh if needed) clicking the toggle button the nav expands and collapses like normal.
BSS3 doesn't have this issue.
Also when Brand is enabled and you inspect the element in Chrome's Dev Tools you can delete the Brand component and the nav toggling still works. So it seems like something in the app upon disabling the Brand component cause the nav toggling to fail.