@McGregor , don't waste your time with the factory reset, it won't help in this case.
We have 5 iPhones in the house, various models. When the battery health is around or under 85%, I replace the battery because it doesn't hold properly anymore. I have already done it in 3 of the phones and another one is coming up, has 85% (2018 model) and I can't play much on it anymore.
I personally think it's because of playing too much on it; you have to recharge it more often, faster discharge/recharge cycles and it ends up shortening your battery life.
This is my personal experience over the last 4-5 years and it only applies to iPhones.
Honest recommendation if I may, buy a battery replacement kit online and change it yourself if able to do so, it costs you about 10% of the money you have to pay to Apple for doing the same.
Be very careful during the replacement operation though, some models are very sensitive!