Haha, I drove into a brick wall with containers and remote NFS mounts. I even wrote a bit of a disjointed article about this on my site. But essentially (I think) “it all boils down to the fact that a containerised OS doesn’t have the same privileges as a non containerised OS. This probably some security restriction imposed by the host OS because the container shares the same kernel as the parent.”
Long story short I also had trouble interacting with remote NFS shares from inside lxc containers.
If this is reproducible without Juju in the mix (which it sounds like it is), it might be better to report an issue with LXD itself? https://github.com/lxc/lxd/