@svantevid Share a copy of your Clover or OpenCore setup, you can redact the Serial Number, Rom, MLB, UUID etc. So I can see what you are using.
Confirm which Make/Model RX 580 you are using. Also confirm which ports you are using and able to see a display, i.e. my PowerColor Red Devil RX 580 has five ports, DP, DP, DP, HDMI & an upper level DVI, as viewed when looking at the rear plate. The Framebuffer I am using shows my RX 580 has just the five connections, when reviewed in IORegistryExplorer.
Rear plate from my Red Devil RX 580, is yours similar, the same?
I can get a display working on the two centre DP, the HDMI and the DVI connections. But I cannot get a display from the far left DP connection, in macOS when using two displays. Works fine in Windows! This is probably a limitation from the AMD Framebuffer. But as I only have the two displays connected I can work with the two DP connections.
We should be able to get yours working the same, but it all depends on which RX 580 card you are using. The playing fields between RX 500 Series cards are not level. Some cards work better than others with the Apple drivers.