You are correct. I re-created the catalina usb and uncheck the auto patcher, then re-install and acceleration works fine, it must be the legacy video card (although I replaced the modded plist file which obviously didnt work).Yes, I experienced the exact same issue last week (I'm still on High Sierra right now), when I was on Catalina. This was before I got OpenCore to work, so I don't think it's related. I was thinking it had to do with Legacy Video Card support tweak within the Catalina Patcher, but I couldn't figure out how to tweak/troubleshoot that. You probably also see some of your video boarders turn black? I don't think it's a "no acceleration" problem, as that would be much worse. It's almost like a broken driver?
I just tried that, but it did not make any difference.
I did get the black screen, i fixed it by replacing the AGC kext.
I am now using OC 0.6.0 and brightness control works for both completely darkness to maximum. I have also copied the OC content into the EFI partition to get rid of the extra USB drive.