Home Assistant removes the deprecated lovelace: mode: yaml setting. Leaving it in place no longer selects ui-lovelace.yaml: Home Assistant logs an error and falls back to a dashboard managed through the interface.
To keep a YAML dashboard, configure it explicitly under lovelace.dashboards.lovelace with mode: yaml and filename: ui-lovelace.yaml. Installations that also define dashboard resources in YAML need resource_mode: yaml. The source includes the complete replacement configuration.