Battery health dropped by 1% in 3 months

What's the problem? Apple rates iPhone batteries for a typical 80% battery health after 500 complete charge-discharge cycles. Granted, everything is simplified into a model where there's a lot of variation, but 1% after 3 months sound normal. I've heard of better than that, but I've certainly heard of worse. Batteries also have manufacturing variations, where I've heard of some that quickly updated to 98-99% while others might have even reported more than 100% (where the value is just the rated value).

Batteries are wear items. They will inevitably decline in battery health.

That's actually pretty remarkable. Most users can expect battery health to decrease 1-2 percent each month.

Batteries are consumable and begin to degrade immediately. Yours is designed to hold 80 percent capacity at 500 charging cycles. Learn more at https://support.apple.com/en-us/HT208387