ITC-308 not staying within in set range

It appears that the sensor will not stay within the range of the set temp of 23c. It just continues to heat and never cool down. I am trying to regulate temp for a hedgehog

Do you mean when the temperature reached TS, the heating indicator of ITC-308 is off, but the heating outlet was still on?
Could you please let me know the voltage and watt of the heater?