There is no proper driver support for the Vega Pro 56/64 graphics cards in iMac Pro Bootcamp. At the moment, Bootcamp is using some kind of generic GPU driver, because the performance is terrible. Most games are unplayable (very low fps, crashes, instabilities etc.). Others who have written about this to AMD and Apple support, have seen replies where AMD blames Apple, and Apple blames AMD. Seriously, this needs to be fixed ASAP.
Thanks for listening,