Suddenly my iPhone battery health became 98%

That can be totally normal as batteries degrade and it depends on your use and charging habits. I have bad habits when it comes to use and charging so my 11 month old iPhone 11 is already down to 86% battery health, but I also got AppleCare+ so when it gets below 80% it will be replaced under warranty.

Because batteries degrage with use, that's just how it is. 98% for a 1 year old phone is phenominal, you have nothing to worry about. When it reaches 80% you should have it replaced.