I put the temperature to 70C on my mini mat and it switches to 25C almost immediately. What's wrong?

When you turn the dial, the first number displayed is the desired temperature. After a second, the display will switch to the current surface temperature.

If you continue rotating the dial, the desired temperature will appear again. Once you stop adjusting it, the display will return to showing the current temperature, which will gradually increase as the mat heats up.