The MSI RX-560 I have (device ID 0x67ff), which is the official Apple recommended version, does NOT work correctly in HS, but works perfectly with Mojave. There are additional drivers in Mojave (which I understand are for that card) that are not in HS. I had previously asked here if anyone had found a way to use them in HS, but no one had. So I may be wrong that HS did not support it with drivers, but they aren't the same in Mojave and have problems in HS. However, I had read previously the there was no officially supported driver from Apple for it prior to Mojave. And it was of course included in the official metal cards list that was issued for Mojave.
I just tested Civ 5 again, and it has the same problem in HS (and NOT in Mojave). The problem only occurs with the game's 1080p full screen mode and not its windowed modes. The screen is a total mess with portions of a double image displaced by about 1/3 of the screen and flickering, etc. It is entirely unusable. There are also rare flashing screen glitches that occur in HS (you may have to wait a while to see them) during normal use of intensive graphics with other programs.
I wrote about this problem almost exactly 1 year ago. Here is the link:
https://forums.macrumors.com/threads/mojave-performance-on-5-1.2144065/#post-26594590
Also see the link in that comment that refers to someone else blaming HS bugs with the same symptoms that also did not occur with Mojave.
https://forums.macrumors.com/thread...-sapphire-pulse-rx-560.2131835/#post-26422177
Anyway, I'm too busy to follow up on this anymore now, so believe it or not.
Also FWIW, I use 2 1080p and 1 4K monitor with the RX-560 using all 3 of its outputs. But I also tested CIV 5 with only a single 1080p monitor connected to the DVI output and had the same problem.