Only some alerts from Prometheus Alertmanager show up on Grafana OnCall

The Prometheus Alertmanager on a Rancher cluster is configured to send all alerts to a MS Teams channel and a Grafana OnCall OSS instance.

All alerts from the Prometheus Alertmanager are seen on the MS Teams channel, but only some of the alerts from the Prometheus Alertmanager are seen in Alert Groups on the Grafana OnCall OSS UI. No error message in the logs of the Prometheus Alertmanager,

What could cause this? Any suggestions on debugging?

THANK YOU!!!