Further info - believe this is something to do with 5.12 kernel and my Ryzen 4300ge. Tried Debian SID with 5.10 kernel (LTS?), no problems. When SID automatically upgraded to 5.12 kernel FFS exhibited exactly the same behaviour as Arch & Manjaro (with the same 5.12 kernel). So the obvious solution will be to roll back the kernel until such time as a more permanent solution presents itself.