Same here
My 6 R9 290 are doing 21-22Mhs
Before they were doing 29mhs
However, this is not connected to ver 12.0
It happened gradually for me for months now
I thought it was connected with overheating because of the fans failing

Maybe it is connected with DAG file size getting larger?

Does anyone know what to do?
Have you tried running -oldkernells?