What the current power mode when you read temperature ? If all sensors are in suspend mode, temperature is disabled.
How long did you read temperature? If the gyroscope is in normal mode(see register 0x03 PMU_STATUS), the temperature is updated every 10ms(+-12%). If the gyroscope is in suspend mode or fast-power up mode, the temperature is updated every 1.28S aligned with bit 15 of the register(0x20-0x21) TEMPERATURE.