iPhone 15 Pro Max battery health reduced from 97 to 94 after iOS 18.3.1 update

Your battery health decline is most certainly not related to the update at all and is just a pure coincidence of timing. You can expect an average of a 1% decline per 40-50 charge cycles, and the decline isn't linear and is subjective to many other factors such as usage and conditions. As lobsterghost1 stated, we will need is how long you have had your iPhone and the number of charge cycles. I really wouldn't worry about your battery health so much until it reaches 80%, when you should replace it. Technology should serve us, not the other way around.

Battery Health goes in one direction only and that is down. On average, you can expect to see about 1% reduction in battery health per month. The decline is not linear, meaning it can remain steady for a while, then drop a few points all at once. That can't be stopped. The question which you need to answer is how long you've owned your iPhone and what the Cycle count is for your phone, which you can check in Settings > Battery > Battery Health.

Xxxxxxvbngffghextra wrote:

It wasn’t like that before the update. I used it about 4 months before 1 reduction . But after the update recently it reduced three times

Updates do NOT and CANNOT reduce battery health. I already explained that battery health decline is NOT linear and it can remain steady for a while then drop a few points.

You didn't answer my question about how many months you've owned the phone or the cycle count? If you don't want to answer that, then there's really not much I can do to help you, IF you even really need any help. For example, of you've owned your phone longer than 6 months, there is simply no issue and you are worrying needlessly over the natural decline in battery health, which as I suggested can't be stopped even if you wanted it to.