Having the exact same issue, going to try and revert the drivers. Very frustrating.
I have the same issue with 18.12.2, 18.12.3 and 19.1.1
My monitors (TN Panels) are locked at 60Hz when I enable Eyefinity.
All my monitors are the same make and size, 2k, 144Hz, Freesync, I use 4k rated displayport cables and two RX580 cards in crossfire.
I installed 18.12.2 on windows 1803 with the same issue, I now have win 1809, same issue.
I have been running Eyefinity at 7680 x 1440 - 144Hz for the past 2 years.
I'm not sure what's going on with AMD I'll just keep running driver 18.10.1 for now.