Why can the POCO F7 Pro only be charged up to 80%?

It is due that you have enabled battery protection or a smart charging protection feature. To extend battery life, the POCO F7 Pro will stop charging when the battery level reaches 80% if you turn on [Battery protection] or [Smart charging]. You can turn off the two switches, which will always allow the device to charge up to 100%.
To turn off the battery protection and smart charging, please refer to the following steps:
1. Go to the [Settings] APP on your phone;
2. Find and tap [Battery];
3. Go to [Battery protection]; 
4. Turn off [Battery protection] and [Smart charging].