3840 × 2160 = 8.2 something MILLION pixels so this means you are oh so close to 2160p. So close that I can't guess why a 8MP shot won't do.
Are there any phones that have a selfie camera that shoot 2160p? According to GSM Arena, even flagships such as the Samsung Galaxy S9+ only shoot at 1440p for the selfie camera:
https://www.gsmarena.com/samsung_galaxy_s9+-8967.php

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