Prometheus Alert Manager: How do I prevent grouping in notifications

I'm trying to setup Alert Manager in a simple setup where it would send one Slack notification for each notification in receives.

I've hoped to disable grouping by removing the group_by configuration.

The problem is, that when I send 2 alert one after the other, even though the Alert Manager shows the 2 alerts as 'Not Grouped' when I get Slack notifications, I get one message for the first alert, and then a second message, where the 2 alerts are grouped.

Here is the config.yml

route:
  receiver: default-receiver
  group_wait: 1s #30s
  group_interval: 1s #5m
#  repeat_interval: 10m
#  group_by: [cluster, alertname]

receivers:
  - name: default-receiver
    slack_configs:
      - channel: "#alerts-test"

Any ideas?