val preview : Preview = Preview.Builder().setTargetRotation(Surface.ROTATION_270).build()

如果setTargetRotation无效哪基本上就是你没指定实现模式。

// viewFinder is a PreviewView instance
viewFinder.implementationMode = PreviewView.ImplementationMode.COMPATIBLE

其实在官方Camerax使用指南就有说明的

Additional controls for PreviewView
Implementation mode
PreviewView can use one of the following modes to render a preview stream onto the target View:

PERFORMANCE is the default mode. PreviewView uses a SurfaceView to display the video stream, but falls back to a TextureView in certain cases. SurfaceView has a dedicated drawing surface, which has a better chance of being implemented with a hardware overlay by the internal hardware compositor, especially when there are no other UI elements (like buttons) on top of the preview video. By rendering with a hardware overlay, video frames avoid a GPU path, which can reduce platform power consumption and latency.

COMPATIBLE mode. In this mode, PreviewView uses a TextureView which, unlike SurfaceView, does not have a dedicated drawing surface. As a result, video renders with blending so that it can be displayed. During this extra step, the application can perform additional processing, such as scaling and rotating videos without restriction.

翻

PreviewView 的其他控件

实现模式
PreviewView 可以使用以下模式之一将预览流渲染到目标 View 上：

PERFORMANCE 是默认模式。PreviewView 会使用 SurfaceView 显示视频串流，但在某些情况下会回退为使用 TextureView。SurfaceView 具有专用的绘图界面，该对象更有可能通过内部硬件合成器实现硬件叠加层，尤其是当预览视频上面没有其他界面元素（如按钮）时。通过使用硬件叠加层进行渲染，视频帧会避开 GPU 路径，从而能降低平台功耗并缩短延迟时间。

COMPATIBLE 模式。在此模式下，PreviewView 会使用 TextureView；不同于 SurfaceView，该对象没有专用的绘图表面。因此，视频要通过混合渲染，才能显示。在这个额外的步骤中，应用可以执行额外的处理工作，例如不受限制地缩放和旋转视频。