My opinion is just an opinion, but I think Apple had better batteries in the past than now. You are not the first one to notice this weird battery behavior, it ages much faster and devices overheat much easier than they used to. It is more relevant to iPhones tho, but if you notice a percent dropping each month and by the end of the year you get something like 88-92% - go to Apple service and tell them device might be defective.
If you already notice weird behavior (overheating, faster battery drain than they claim) you can already give Apple a visit.
After all, if it is “normal” to lose 10% of battery life every year. Doesn’t it mean that device’s effective life will end in just two years and a half?
This doesn’t compare at all to my iPad Air 2. While I don’t use it nowadays, it still works and has enough battery to watch one big movie. Device was released in 2014. In 2019 it still had enough juice to watch 3 full movies each 2 hour long