Possibly could be because of many of the few reasons like charging cable phisical damage, charger at fault may be some internal damage within. Maybe your phone's charging port have some sort of physical damage or the software side issue that causing not to properly read the battery capacity. Check with another branded cable, charger and both the charging ports. The bottom and the side ones. While charging check if your charger is getting a bit warm or a bit hot if it's unusual maybe need to change carger and cable. Same case with the port, try to charge with side port with the stock cable and 30w charger if you feel your phone getting more hot than little more than warm. That's unusual could be caused due to something wrong within motherboard charging module. As per personal experience Asus Rog phones battery doesn't degrade or acts as you mentioned in just a years time. So most likely your battery won't be having any possible issues. Only the software or the motherboard could be the problem causing unusual battery percentage reading.
