Alertmanager v0.28.1 not showing proper data

[druanoor]

Hi.

After updating from 0.28.0 to 0.28.1, alertmanage does not show accurate data about active alerts.

We manage all alerts from ThanosRuler instance (except a Prometheus Hearbeat which is managed by Prometheus) and the only alert that show up on the Alertmanager is the PrometheusHeartbeat

Ui and amtool show the same output:

./amtool --alertmanager.url="http://localhost:9093" alert
Alertname            Starts At                Summary    State
PrometheusHeartbeat  2025-04-22 23:37:05 UTC  heartbeat  active

The strangest thing is that alerts triggered by Thanos Rulers are flowing through alertmanager and reaching their end destination, but without being listed as active on AM.

Any ideas?

Thank you