This situation is normal, due to the charge management function of Apple's MagSafe external battery. Both the MagSafe battery case and the iPhone can heat up when charging because they're left plugged in for a long time. Therefore, in order to extend the service life of the battery, the charging may be limited to more than 80%, and the power may be automatically cut off when it is charged to 80%.
If this happens, Apple officials recommend moving to a cool, ventilated place, when the temperature drops, it will be recharged.
Because the charging speed is relatively slow, it causes the charge to be dissatisfied