To give some context to my setup, I have a GTX 770 controlling my main monitor and an ATI Redwood series card controlling my secondary monitor. The GTX 770 has given me no issues at all and works as intended. After installing Solus 3, both cards/monitors worked completely fine, but after updating all packages my second display no longer works.
I have tried uninstalling all video drivers except for the proprietary nvidia driver, reinstalling xorg-driver-video-radeon. After updating the proprietary nvidia driver a few days ago, my secondary monitor displays the text login screen for a second during boot, but then only shows a blinking white cursor in the top left which freezes when the greet screen is displayed, which didn't happen before (simply no display at all).
Here is a dmesg result with a search for "radeon" which doesn't seem to be showing any issues. : https://pastebin.com/VjNdWVi6
lspci result: https://pastebin.com/S6QrYyuB
EDIT: Pressing ctrl+alt+f2 shows the terminal on my second display, it is working. I can't get any actual graphical display there. I have the radeon driver installed as well as vesa. The second display only works AT ALL whenever the proprietary nvidia driver is installed. Very weird situation.