iPhone 12 Battery health drops unusually fast

Hi! So I’m having an issue with my iPhone battery health. I sent my phone to have the battery replaced(by an authorized service provider) in the middle of January this year and got it back at the end of January. And when I got it back, initially, the battery health was at 100% for about 5 - 6 months, when it dropped to 99%. And about 2 weeks later, when I updated to the 3rd dev beta of iOS 26, the battery health started dropping very fast. Since the 20th of August, I started keeping track of the drain. Here it is:

August 20 - down to 92%

August 25 - down to 91%

August 30 - down to 90%

September 3 - down to 89%

September 9 - down to 88%

September 14 - down to 87%

September 19 - down to 86%

September 22 - down to 85%

September 26 - down to 84%

As you can see here, every five days on average, the battery health is dropping by one percent. And according to my projections, at the very end of this month, the health will be 83%, the same amount that it was at when I got this iPhone 12 secondhand in October last year.

And this makes me worried. Will it be dropping that fast, or is there a way to fix this(maybe some battery recalibration)? I know that I could just send the phone again, but I would really like not to do that, because I’m saving up money for an Apple Watch!

Thanks to y’all in advance for help! Have a nice day!

PS. I’m currently on the public final release of iOS 26.0, but I have an update to 26.0.1 ready, so by the time I return to this post, I’ll be at 26.0.1!

EDIT: Changed some wording