If I shrink the browser size until the menu turns to the mobile hamburger menu, then the primary widget area does disappear.
What device are you having a problem with (tablet, phone, Ios, Android….), and does the device display the full menu, or the mobile hamburger menu.
Mobile is defined by the screen resolution. So on certain device size / resolution, it is possible, the full menu is displayed, indicating it is not considered a mobile device.