Battery health decreased into 99 after 26 days in 11 pro

Without intending to seem flippant, stop trying to find problems. 99% is just fine. 98% is just fine. 86% is fine.

80% is heading for a replacement.

What to do? Use your iPhone for what you purchased it for.

The battery here will degrade, inevitably. All batteries degrade. Always. Inevitably.

What’s new here is that you have access to a visible measurement of this degradation.

If (when?) your battery gets to ~80% or gets too short for your usage without added recharging, battery replacement services are available. This if you’re not going to replace the whole iPhone by then.

Here is what Apple suggests for longevity: https://www.apple.com/batteries/maximizing-performance/

What else to do? Keep current on your iOS software and apps, keep to the recommended temperature ranges, and enable the battery health optimized-charging support, if that’s not already enabled

Yes, that happens. Batteries degrade over time no matter how they are used. My iPhone XS dropped to 93% in a few months, and since then has stayed at 90% for almost two years.

Your iPhone 11 may be new to you, but it may have sat on a shelf for many months.