My farm is a mess with v12.0 - i'm moving back to v11.1 until claymore sorts out all the issues with crashing GPUs etc.
The strange part is that it seems v12.0 actually messed up my Fury cards somehow so now when I revert to 11.x they still crash. Could be some kind of odd coincidence but it seems unlikely.
You've probably already tried this, but have you powered off the rig, disconnected the power cord for 15 seconds or so (to ensure the PSU drains residual current), and power on? It sounds as the the GPUs have not been fully reset after you switched back. Just a thought.