I think I find solution. It's happen because MIUI have 2 settings app with 2 different power saver. My method to access another settings is

1. Download Shortcut Maker By Rushikesh Kamewar: Google Play

2. Open the app. Go to Activities. Search for saver.

3. Select Battery Saver under the Settings. and choose
Try this Shortcut

4. If you see Turn off when Charged in enable position, turn that thing off. Thats the source of the problem.

Let me know if this helpfull. Sorry for my bad in english speaking.