MSI Afterburner Core Clock setting is ignored

[wilkas]

Have MSI RX480 GPU. Have set fan speed to custom curve. Have set core clock to 800 MHz. Use NiceHash Miner for mining. It seems that for a while core clock sticks to the limit of 800 (fan works silently), but later it starts to ignore the limit and go as high as 1200 causing fan to work loudly.

Why does it happen and how can I set it stick to lower core clock (I need silent fan during sleep)?