I’ve used my iPad charger , my MacBook charger with a usb c to lightning cable and another cable with an output of 2Amp and it is either losing a charge or steady if I’m using the phone at all.
80% on or off the charger? I don't have an X, but any iPhone I ever had that did like that, was an app or two misbehaving. I swipe out any suspect apps out of multitasking, then restart my phone. Then let it sit a few minutes.
If on charger, then the ESPN app (using screen, CPU, GPU, audio) constantly for two hours could have been consuming the same power that was going in while on charger.