Apple have to request a driver update from AMD before AMD go about testing a newer version for Macs.
If enough people complain to Apple then they will request another driver update if it is unusable for you.
There is an unofficial route also at www.bootcampdrivers.com which is the website I run.
18.8.2 works well on Vega 64 but doesn't work at all on Vega 56. The last working version WAS 18.5.2 until the latest Windows update broke it.
Now a workaround method for 18.3.4 is the last known working version for Vega 56.
18.9.1 has not been tested yet for iMac Pro though I will be releasing Alpha status drivers in my forum for willing volunteers to test today.