If your iPhone sometimes only stops charging at 80%, it is most likely due to a feature Apple has implemented to extend the life of your iPhone battery. Apple has introduced a battery optimisation feature in iOS 13 that slows down the charging of your iPhone and even keeps it at 80% to prevent excessive battery drain.

To turn off Optimised Battery Charging, go to Settings > Battery > Battery Health and turn off Optimised Battery Charging. You will see two options: Disable until tomorrow or Disable (permanently). Choose which you prefer.

I think is a problem/situation directly with iPhone. I use it to charge my Android (OnePlus6T - 3700mAh) and my girlfriend's iPhone 12 (2800 mAh), her phone can't be charged to 100% but mine can be charged to 100% several times.