In short, if anyone finds this issue occurring:
weaviate images are built on Alpine Linux which uses the musl DNS resolver, rather than the more standard libc based one.
musl can behave weirdly when DNS isn’t configured specifically for it in the K8s environment.
The solution to the problem is for weaviate to only use FQDNs for communicating between nodes, which in principle means changing all the CLUSTER_JOIN environment variables to be weaviate-headless.{{release.namespace}}.svc.cluster.local. << The extra . on the end is the fix.