I've gotten my FE+RX crossfire setup to work via the above process (my own). This runs fine in several games, Prey being one.
BF1 however, will launch in CF displaying the CF logo properly on a black screen and then switches to a corrupted display over the 3 portrait monitors at 4320x2560x144. It then alternates between these two continuously. However, I think the game is still processing in the background, not locked up, but testing this has been challenging. Note: the same behavior occurs for all other resolutions and refresh rates in eyefinity as well, though a portrait aspect low res version duped over 3 monitors with filckering has worked before, so I suspect the problem is due to video memory in some way.
I've had similar corruption where the screen would stall to less than 1fps when in exclusive fullscreen from a single RX 480 using eyefinity, but I think this was due to the eyefinity configuration having swapped monitor numbering vs the windows numbers.
I can run in fullscreen for the eyefinity display with just the Vega FE now that I've fixed that. But Crossfire must be disabled entirely, not just in the profile for BF1.
This is literally an edge case that is one atom thick. I might be the only FE+RX CF Eyefinity user on the planet.
But I wanted to let you know. Also, I own some stock. Heh.