Have you found a solution?

Setting extendedLayoutIncludesOpaqueBars = true in my viewController doesn't work.

Is your viewController containing a UIScrollView? I discovered there's a subview of type _TtCE5UIKitCSo15_UIScrollPocket12BackdropView in UIScrollView, and by setting its backgroundColor to nil, the extra backgroundColor disappears, allowing content to extend behind the TabBar. But again, this approach feels too hack.

I tried to find where the backgroundColor was being set to an actual color value, but found nothing.

Strangely, my tabBarController has four viewControllers - three tabs exhibit your issue, but one tab's viewController has no occlusion and extends properly... yet I can't find any different property setting that would cause this behavior...