I can confirm this issue exists on rx560

i have discovered a workaround, 

if you copy the stock powerplaytable to a file with
cat /sys/class/drm/card0/device/pp_table > pptable

then load the stock pptable with

sudo bash -c 'echo pptable > /sys/class/drm/card0/device/pp_table'

the flickering goes away without having to lock the mclk to the highest state

it may also be related to https://bugs.freedesktop.org/show_bug.cgi?id=110347
this bug.
by default at boot the voltage for the highest pstate is the maximum allowable
voltage(1150mv) not the powerplay defined voltage of 1025mv, all other pstates
are not effected

this is also corrected after reloading the pptable manually.