This behavior is expected and correct. It is intentional for smooth charging, especially at low voltages. The IC itself is controlling this behavior, so it is nothing that the GUI or other host needs to manage.
When charging, the nVPrtTh1 value is saved to the Back up location and the nVPrtTh1 value is written to the minimum value. When charging is not detected, the value from the Backup location is refreshed to the nVPrtTh1.