Storage on Kubernetes can feel like a bit of a dark art. Juju does have a pretty good story here, but it can take a little bit of time to get a firm mental model about what Juju is doing under the hood.
I seem some recall that there are some known issues with deploying Kubernetes onto LXD.
Here are some links that might provide a little bit of guidance while someone with some more expertise (pinging @hpidcock @wallyworld) is able to get some time to help you out.