
The POCO F7 Ultra, one of Xiaomi’s premium devices, is designed with cutting-edge hardware and features that should easily handle video playback in full-screen mode. However, many users have reported unexpected random reboots when playing videos in full-screen.
Xiaomi has acknowledged the issue and is currently investigating the root cause. Progress has been slow, but a fix is expected in a future OTA update. Until then, users on the global ROM are advised to avoid installing the latest build.
Details POCO F7 Ultra random reboot during video playbacks

According to the latest HyperOS weekly bug report, global POCO F7 Ultra users running firmware version OS2.0.202.0.VOMMIXM are affected. Playing videos in full-screen consistently triggers a reboot. Since the device is otherwise capable of handling supported video formats, this behavior is confirmed to be a software bug.
The issue is limited to global ROM users, and no similar cases have been reported on other variants. Xiaomi is working on a patch, but until the update arrives, users are advised to avoid full-screen video playback to prevent random restarts.
source: Mi Community