Apple states in their support documents that optimized charging may occasionally charge a little higher and a little lower than the set charge level. In addition it will occasionally charge to 100%. This is necessary for charging algorithm to learn what 100% actually is. Think about it. How can it charge to 80% if it doesn’t know what 100% is.
>>If you have Charge Limit set to less than 100 percent, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.<<
About Charge Limit and Optimized Battery Charging on iPhone - Apple Support
The iPhone has to do this so it can calibrate to 100% battery capacity. It’s normal.