That's the problem. A very long time problem without solution.
I have my monitor displayport (1080p 75 Hz) and mi TV by HDMI (1080p 60 Hz). When I disconnect one of the screens the clocks go down to 300/300 MHz. But when I connect the 2 screens then the memory clock go up to the max. The result? 25º degrees more in iddle.
Temps: 29/30º iddle with 1 screen
55/56 iddle with 2 screens
GPU: MSI RX 570 Armor 4 GB (but happen with all AMD GPU's)
Please AMD fix that. Work harder or change your drivers developers