Nope! Although, if it doesn’t meet the temperate set it will still keep going! It shouldn’t be breaking that temperate barrier. If it is, you’re best to make a warranty claim. Dyson offer a 2-year cover. They’ll either repair or replace it for you!!

Extract from - Using Auto mode (Purifier, Heater and Humidify) | Dyson Community
 

Heaters

When in Auto heating mode, your machine's on-board sensors will intelligently adjust the settings according the air quality and the temperature level you had set. 

If the machine detects a drop in the air quality, the airflow speed will increase until the air quality target has been reached.

If the machine detects a drop below the temperature you had set, the airflow speed will increase until this temperature have been reached.

In the event that the machine detects a drop in the air quality and the temperature, it will increase the airflow speed until the target air quality and temperature have been reached. This way your purifier won't be constantly running. 

If it isn’t working as stated above, it’s faulty for sure!