I think this may be because of a different game version between the phones, not saying it isn't fully updated but as the S21 is so new asphalt may be yet to optimise asphalt 9 to take advantage of the tech. This is the case with some android phones at the minute as the they have a different more intensive version of the game on high end phones so it should be coming to your s21 but may not be ready yet. Sorry it's not a perfect answer but I hope you find a solution.
I bought the S20 Ultra also, such a disappointing phone. Totally get what you mean about it getting hot but the battery life was my biggest issue. I sold it and switched the the Oppo Find x2 Pro which I really like. I do love my tablet though .