From the Apple article:
About 80% Limit with iPhone 15 models
With iPhone 15 models, you can choose between Optimized Battery Charging, 80% Limit, and None.
When you choose 80% Limit, your iPhone will charge up to about 80 percent and then stop charging. If the battery charge level gets down to 75 percent, charging will resume until your battery charge level reaches about 80 percent again.
With 80% Limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.
This implies that it is around 80% not necessarily exactly 80% so it may still be working as intended. You could report it to Apple if you think it is a bug and not functioning correctly though
About Optimized Battery Charging on your iPhone - Apple Support