I use a mixed triple screen setup with a center 24" LG 24MP68VQ-P (1080p + FS) and a 17" to each side (1280x1024). I use active DP-> VGA Adapters and everything worked fine with Eyefinity (17.3.3) set to "expand", so I got full 1080p on the center screen and 1:1 pixelscale on every screen.
-> Using 17.4.4 and now 17.5.1 changing eyefinity mode from "fit" to "expand" the resolutions changes from 4480x1024 to 4480x1080 but I still got the black bar in the center screen and the pixel scale is not 1:1 anymore. I guess it looks like the 1080p picture is squeezed to a height of 1024 px.
I guess there is no workaround until the bug is fixed in a new crimson version?
2*16 GB DDR4 GSkill @ 2666
Powercolor RX 480 Red Devil
Win 10 Pro Creators Update with crimson 17.5.1 and newst AMD chipset drivers (Ryzen balanced Powerplan)
I was having the same problem, got sick of waiting, and dug into it. I made the primary eyefinity monitor the smaller side monitor instead of the bigger main monitor and it worked like normal.
Hope this resolves it for you too.