The HDR effect you’re experiencing is controlled by the Adaptive HDR function, which is designed to optimize HDR display effects based on various factors such as composition and light sensing. When the video is in DEVICE composition mode, changing the light input will trigger the Adaptive HDR function, enhancing the HDR display for a better viewing experience. However, when switching to full screen, the light sensing input can be disrupted, causing the HDR effect to disappear. If the Adaptive HDR function is not activated, it will not impact the original HDR quality of the video, and the video will play without the enhanced HDR effect.