I’m experiencing exactly the same issue on both my iPhone 16 Pro and iPhone 17 Pro after upgrading to iOS 27.
Interestingly, a similar issue occasionally occurred on iOS 26, particularly when using the native Camera app in Video mode while screen mirroring to Apple TV. However, I was always able to work around it by switching from Video → Photo → Video. This would correctly rotate the mirrored camera view into landscape orientation.
Unfortunately, since upgrading to iOS 27, this workaround no longer works at all.
The Camera app’s interface appears to rotate correctly when I turn my iPhone horizontally, but the mirrored display on Apple TV remains stuck in portrait orientation. Other apps, such as Safari, rotate correctly without any issues.
What’s particularly interesting is that Blackmagic Camera works perfectly fine in the exact same setup. When I use Blackmagic Camera and mirror my iPhone to Apple TV via AirPlay, the TV display rotates correctly into landscape orientation, exactly as expected.
This strongly suggests that the issue is specific to Apple’s native Camera app rather than AirPlay, Apple TV, or the iPhone’s orientation settings.
I personally prefer using Apple’s native Camera app, especially for its video quality and Dolby Vision HDR capabilities. However, I’m now forced to use Blackmagic Camera simply because it handles landscape screen mirroring correctly.
I really hope Apple investigates and fixes this issue in an upcoming iOS 27 update. Landscape camera mirroring is an important feature for my use case, and losing functionality that previously worked is extremely frustrating.