Why is the room temperature shown by the Daikin online controller different from the set temperature even after the room temperature has already been reached?​

This is normal behavior of the unit.

The temperature displayed on the Daikin Online Controller is the one detected by ther room temperature sensor located inside of the indoor unit. This temperature can differ from the actual average temperature in the room because of several effects. For example, by laws of nature hot air rises, so the upper air layers of the room will be warmer than the average temperature. The difference in temperature will depend on the size and heat losses of the room, unit installation position in the room, its distance to the ceiling and the fan speed setting.

The unit's control logic is designed to compensate for this effect. It controls the average room temperature and keeps it as close as possible to the temperature set by the customer.