Dynamic Dashboards (dashboardNewLayouts) breaks provisioned dashboards on save - OSS 12.4.0

### What happened? When enabling the `dashboardNewLayouts` feature toggle in Grafana OSS 12.4.0 (Docker), saving a provisioned dashboard causes the dashboard to render blank after reload. No errors are shown in the logs. ### What did you expect to happen? Dashboard should save and continue displaying panels normally. ### Did this work before? Grafana OSS 12.3.4 ### How do we reproduce it? 1. Run Grafana OSS 12.4.0 via Docker with provisioned dashboards (JSON files) 2. Enable feature toggle: `GF_FEATURE_TOGGLES_ENABLE=dashboardNewLayouts` 3. Restart Grafana 4. Open a provisioned dashboard 5. Make any edit and save 6. Reload the dashboard ### Is the bug inside a dashboard panel? No ### Environment (with versions)? - Grafana version: 12.4.0 - Installation method: Docker (`grafana/grafana:12.4.0`) - OS: Linux - Feature toggle enabled: `dashboardNewLayouts` - Dashboards: provisioned via JSON files in `/etc/grafana/provisioning/dashboards/ - Browser: Chrome ### Grafana platform? None ### Datasource(s)? _No response_