I am also running into the same issue. I have two questions that might help
tracking down why we are having issues, but not all people that are running a
Vega graphics card.

1)

What is the output of the following command for you guys?

cat /sys/class/drm/card0/device/vbios_version 

I am running the following version:

113-D0500100-103

According to the techpowerup GPU bios database, this is a vega bios that was
replaced two days (!) later by a new version. Perhaps issues were found that
required another bios update? I might install Windows on a spare HDD and try to
flash my Vega to see if that changes anything.

2)

Memory clocking is different for people running multiple monitors. Are you guys
also running multiple monitors by any chance?