I think the bug is in linux, where it didn't provide a way to not create an IPv6 socket; specifically the /proc/fs/nfsd/portlist interface can only be to control transport protocol (tcp, udp, rdma) and port number. The underlying code of this interface is hardcoded to create both IPv4 and IPv6 sockets: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/nfsd/nfsctl.c?h=v6.10#n718
- Bug#648635: May be it is out of nfs-utils' control WHR
