Repeated notifications sooner than `repeat_interval`

What did you do?
Created an alert and a route to Slack with the following config:

group_wait: 10s
group_interval: 5m
repeat_interval: 24h

What did you expect to see?
One alert every 24h for the same alert

What did you see instead? Under which circumstances?
A repeated alert being sent after some amount of group_interval has passed (5m, 10m, 15m, etc). What I believe is happening is that a new data point appears after some amount of time < group_interval, but it then has resolved itself after the full duration of the group_interval. Additionally I have confirmed that there are no changes in the labels for the alert; I have viewed the query that triggers the alert and the labels stay consistent the whole time.

For example, we have an alert that is triggered when a kubernetes pod is in the state: Error:

• at 10:50.00 an alert is triggered with the labels pod: foo, state: Error
• at 10:50.10 the alert is sent to Slack after waiting the group_wait
• at 10:52.00 a new data point appears with the labels pod: bar, state: Error. An alert is not sent yet because the group_interval of 5m has not passed yet.
• at 10:52.30 the pod: bar is no longer in state: Error (note, pod: foo is still in state: Error
• at 10:55.10 the alert is sent out but it only contains the info for pod: foo, state: Error because the pod: bar is no longer in state: Error

I would expect to see either:

• No new alert being sent out at 10:55.10
• A new alert being sent out at 10:55.10 that contains info about pod: bar

Environment

• System information:

$ uname -srm
Linux 5.4.38-17.76.amzn2.x86_64 x86_64

• Alertmanager version:

$ alertmanager --version
alertmanager, version 0.21.0 (branch: HEAD, revision: 4c6c03ebfe21009c546e4d1e9b92c371d67c021d)
  build user:       root@dee35927357f
  build date:       20200617-08:54:02
  go version:       go1.14.4

• Prometheus version:

$ prometheus --version
prometheus, version 2.19.2 (branch: HEAD, revision: c448ada63d83002e9c1d2c9f84e09f55a61f0ff7)
  build user:       root@dd72efe1549d
  build date:       20200626-09:02:20
  go version:       go1.14.4

• Alertmanager configuration file:

global: {}
receivers:
- name: slack-receiver
  slack_configs:
  - api_url: <url>
    channel: <channel>
    send_resolved: true
route:
  group_by:
  - alertname
  group_interval: 5m
  group_wait: 10s
  receiver: slack-receiver
  repeat_interval: 24h
  routes: []

Start with Alertmanager's notification scheduling around group_interval and repeat_interval, using the reported pod label lifecycle as the reproduction case. Trace why a resolved alert group member causes a notification at 5-minute intervals; done means unchanged labels do not trigger an early repeat and the notification behavior matches one of the stated expectations.