You should only need to change the Host if your upstream address is HTTPS and the upstream expects the Host to match TLS-SNI.
Caddy sets the Host header automatically to the original incoming host (so in your case, grafana.home) so it shouldn’t be necessary to change.
Enable the debug global option and see the proxy logs, you’ll see the Host header’s value.
Are you sure your Grafana instance is configured to expect grafana.home or is it expecting something else?