Can confirm. I'm using a Rx 560 on kernel 4.16 and my clock rate is stuck on
1230MHz permanently.

I'm also using a single 2560x1440x144 monitor

This is directly related to DC. If I run with amdgpu.dc=0 the issue disappears.
If I run with amdgpu.dc=1 it shows up. I can't say if it also happened on 4.15
or earlier.

I did notice that dmesg shows about a dozen powerplay errors:

[   94.582083] amdgpu: [powerplay] 
                failed to send message 146 ret is 255 
[   94.582091] amdgpu: [powerplay] 
                failed to send pre message 147 ret is 255 
[   95.895976] amdgpu: [powerplay] 
                failed to send message 145 ret is 255