That’s strange indeed. I have VirtualBox 6.1.14 as well (vboxmanage --version
says it’s 6.1.14r140239), for me vboxmanage help modifyvm
lists these options: Am79C970A|Am79C973|Am79C960|82540EM|82543GC|82545EM|virtio
I doubt this has so much to do with VirtualBox version, but it’s about your specific setup. It might be related to the kernel version as well - the VirtualBox folks did a lot of work to get VirtualBox even working on the new kernel, and I wouldn’t be surprised if there’s still a bug or two left. I suggest you file a bug in the VirtualBox bug tracker