Thank you for your continued support of Xiaomi products. The issue you're experiencing occurs because the third-party application in question does not support video calls while in split screen mode, particularly when using vertical orientation. This limitation is related to how the application handles multitasking and video call functionality simultaneously. To ensure the best experience during video calls, it is recommended to use the application in full-screen mode, without splitting the screen. This will allow the device to fully allocate resources to the video call, ensuring smooth functionality and better overall performance.