iPhone 12 Pro Max Battery Health

Well, when you start a smartphone company you can set the warranty limit at 90% if you want. Since no smartphone companies actually make their batteries they go by the recommendations of the battery makers. All battery makers say the battery is fine until it reaches 80% health/500 charge cycles. So all the smartphone companies (including Samsung, Google and Huawei) use that for warranty purposes.

A battery is still fine at 87%. And battery health does not go down in a linear manner.

OK, let me know. Just realize the battery maker will not cover it under their warranty so you will be absorbing the cost for millions of batteries that don't need to be replaced plus you are going to have to find an ecologically sound method to dispose of the batteries you replace. That will also cost a LOT of money. You are going to need to charge about $500 extra for each of your phones.

They are telling you the truth, that is normal. They usually will not replace the battery at all until it reaches 80%

Cover what? Your battery is at 87%. That is fine. If it was 80% or lower at 11 months they would replace it.