This is a manufacturing defect of exynos 9611 soc and a quality control fault of Samsung They had provided free repair to many users but now it has become widespread https://r2.community.samsung.com/t5/Tech-Talk/Auto-Restart-Brand-solution/td-p/10333335 Follow the above steps in the post, if they deny, then ask for discount and be adamant that the update has damaged your phone, tell them you will file complaints in consumer forum
