While the S21 series is supposed to launch a month earlier than expected, it seems that the design is still being tweaked. OnLeaks is back with a revised version of the Samsung Galaxy S21 Ultra, which has a different camera design from the original renders.

No, the camera island will still look like that, but there’s an extra module on the back. It’s not clear if this is a fifth camera or some sensor (a 3D ToF sensor, perhaps?).
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https://www.gsmarena.com/samsung_galaxy_s21_ultra_renders_revised_now_theres_an_extra_camera_or_sensor-news-46031.php