Good morning! Last night I opened a pull request to fix the migration of templates from old to new alerting. You can find it here Alerting: Migrate old alerting templates to use $labels by grobinson-grafana · Pull Request #62911 · grafana/grafana · GitHub.
If I remember correctly the reason it doesn’t work in Classic Conditions is because when you have multiple series, the classic condition will pick which ever series is exceeding the condition. This means that $labels can change from one evaluation to the next without the original alert resolving and a new alert firing.