Battery health dropped to 90% in 8 months

On average, Battery Health will decline at a rate of about 1% a month. Might be a bit more, or a bit less.

So, your battery is still in the normal range, but you are.probably using the iPhone frequently.

5G really eats up the power if you are using that with you carrier....and a weak cellular signal uses up a lot more power than a strong signal.

Should also be said, once it hits 80% or below, you should replace the battery.