Yes, genuine Sapphire Pulse RX580, 8GB. I have tried a multitude of things, including reset PRAM, reset NVRAM, swap the card to PCI x16 slot 2, swapped out display cables, and updated firmware on the monitors. I've searched through the monitor menus and I cannot find anywhere to toggle between DP 1.1 and 1.2. Seems it is just DP. Funny thing is that it worked on DP just fine prior to the 10.13.6 update. I have read in another online post where someone was experiencing a similar issue. Their solution was to just stick with HDMI, which is OK. But I thought I'd bring the issue here to see what all the experts might have to suggest.
Again, I can live with HDMI only. I just find it odd that this is happening. Here's what shows on System Report.
Radeon RX 580:
Chipset Model: Radeon RX 580
Type: GPU
Bus: PCIe
Slot: Slot-2
PCIe Lane Width: x16
VRAM (Dynamic, Max): 8192 MB
Vendor: AMD (0x1002)
Device ID: 0x67df
Revision ID: 0x00e7
Metal: Supported, feature set macOS GPUFamily2 v1
Displays:
DELL UP2718Q:
Resolution: 5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
UI Looks like: 2560 x 1440 @ 60 Hz
Framebuffer Depth: 24-Bit Color (ARGB8888)
Display Serial Number: 73K0276MCXXX
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
DELL UP2718Q:
Resolution: 5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
UI Looks like: 2560 x 1440 @ 60 Hz
Framebuffer Depth: 24-Bit Color (ARGB8888)
Display Serial Number: 73K0279BCXXX
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No