Battery Health is getting weaker after the new update

No, the update may have more accurately calibrated your battery health, but the update itself is not capable of making battery health decline. Battery Health declines with every full cycle battery charge. It will continue to decline whether you like it or not. And when it finally declines to 80%, it will then be time to pay Apple to replace the battery in your iPhone.

Again, Battery Health didn't get "weaker" as you suggest because of an update.