iPhone is charging upto 80% only, even after disabling the optimise battery charging option also

With iOS 13 and later, Optimized Battery Charging is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged. When the feature is enabled, your iPhone will delay charging past 80% in certain situations.

About Optimized Battery Charging on your iPhone - Apple ...

Try calibrating the battery:

https://www.howtogeek.com/296288/how-to-fix-inaccurate-charge-percentages-by-calibrating-your-phones-battery/

https://www.makeuseof.com/tag/calibrate-iphone-battery/