Do you use cp -p? It's not unexpected for write permission to be dropped on non-owner otherwise. Check umask on your client. For example, mine is 022 which would cause 777 to become 755 on copy. Some clients may also be more aggressive at dropping mode bits on a remote copy.
OK, can we close this as working as designed?
I don't think we need to do anything special. This is the Linux command working as expected. It isn't NFS speciic.