Are you comparing the motion apples to apples, as in on the exactly the same pages or sites? Ultimately, since there is no setting, which shows whether you are on 120hz or not, if you are convinced there is an issue, you should schedule an appointment at your Apple Store Genius Bar and let Apple see the phone. Since it's an iPhone 13 Pro, which was discontinued in September 2022, your phone is most certainly no longer under warranty. If a repair is required, you'll have to pay for it. I guess the question is how important is this to you? Another question would be if the phone has ever had the screen replaced? And if it was, was it replaced by Apple itself or an Apple Authorized Service Provider, like BestBuy in the US? If it was replaced by anyone other than Apple or an Authorized provider, you likely don't have a Promotion Display on the phone. Only Apple or an Authorized Provider have access to authentic Apple Parts. If this is your situation, there would be no point in taking it to Apple. They won't touch a phone with aftermarket parts installed.
Finally, did you buy the phone brand new from Apple when they still sold them? Or perhaps, did you buy a used iPhone, which could be refurbished, not by Apple.