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This is by design, and the built-in MRC will stop when your application requests ExclusiveControl access to the P/V camera. Please refer to Mixed reality capture (DirectX) - Mixed Reality | Microsoft Learn
If an app requests ExclusiveControl access to the photo/video camera, built-in MRC will automatically stop using the photo/video camera so the app's request will succeed.
However, you can try to implement the custom video capture function with SharedMode as SharedReadOnly through the MediaCapture API. Regarding the MediaCapture API, please refer to Basic photo, video, and audio capture with MediaCapture - UWP applications | Microsoft Learn.
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