One of the many developers on XDA found some interesting text strings in the camera app on the new MIUI 11, strings that clearly refer to the possibility of recording video in @ 8K ☺30fps.
name="pref_video_quality_entry_8kuhd">UHD 8K, 30fps name="video_ultra_clear_tip_8k">8K UHD
7.680 × 4.320 pixels to be precise, the profile found; the point now becomes to understand which smartphone it refers to.
None of the current models should in fact be able to record with a similar resolution and framerate. Mi MIX Alpha's new 108-megapixel sensor also stops at videos in 6K @ 30fps, and apart from that the image processor of the Snapdragon 855 does not go beyond 4K @ 60fps. Technically it would be possible to push it up to 8K, but with no more than 15/24 frames per second and with poor quality, especially since digital stabilization is in no way supported at this resolution.
The only reasonable hypothesis is therefore that we need a new flagship SoC, or the successor to the Snapdragon 855/855 +, which will not see the light before next year. The details about this new chip are still scarce, so this remains at most a reasoned hypothesis, which we will verify over the coming months. In any case, Xiaomi seems safety of the fact that a smartphone based on the MUI 11 will be able to record video in 8K @ 30fps; just don't hold your breath waiting for its advent.