Repeated resolved alerts after 'repeat_interval'

After updating alertmanager from version 0.19 to 0.20, I had a problem. When I group alerts, the resolved alerts are not disappear anymore in the case that there are already firing alerts in the group. When upgrading to version 0.21, the problem is still reproducible.

It seems this problem appeared after PR#2040.
In the annotation to the PR, it was written that the aggregation group is responsible for removing the resolved alerts. However, this does not happen - perhaps I've missed something.

What did you expect to see?(That's how it worked before):

• I get 3 firing alerts
• After sometime I get 2 resolved alerts - So there are 1 firing alerts and 2 resolved alerts
• After repeat_interval, there are 1 firing alerts

What did you see instead?

• I get 3 firing alerts
• After sometime I get 2 resolved alerts - So there are 1 firing alerts and 2 resolved alerts
• After repeat_interval I get 1 firing alerts and 2 resolved alerts. Nothing changes.

Environment

• System information:

  Linux 4.15.0-115-generic x86_64

• Alertmanager version:

  alertmanager, version 0.20.0 (branch: HEAD, revision: f74be0400a6243d10bb53812d6fa408ad71ff32d)
  build user: root@00c3106655f8
  build date: 20191211-14:13:14
  go version: go1.13.5

• Prometheus version:

  prometheus, version 2.18.1 (branch: HEAD, revision: ecee9c8abfd118f139014cb1b174b08db3f342cf)
  build user: root@2117a9e64a7e
  build date: 20200507-16:51:47
  go version: go1.14.2

• Alertmanager configuration file:

global:
  resolve_timeout: 3m
...

route:
  group_by:
  - environment
  - alertname
  - severity
  group_wait: 20s
  group_interval: 80s
  repeat_interval: 3m
...

• Logs:

Feb 02 02:49:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:49:44.364Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[1d2cb32][active] 
Feb 02 02:49:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:49:44.364Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[281d27a][active] 
Feb 02 02:49:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:49:44.364Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[4f37afb][active] 
Feb 02 02:49:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:49:44.365Z caller=dispatch.go:473 component=dispatcher aggrGroup="{}/{severity=\"warning\"}/{environment=\"testing\"}:{alertname=\"DiskReplace\", environment=\"testing\", severity=\"warning\"}" msg=flushing alerts="[DiskReplace[4f37afb][active] DiskReplace[281d27a][active] DiskReplace[1d2cb32][active]]"
Feb 02 02:49:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:49:44.530Z caller=notify.go:685 component=dispatcher receiver=alerts-testing integration=webhook[0] msg="Notify success" attempts=1
Feb 02 02:50:29 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:50:29.364Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[281d27a][resolved]Feb 02 02:50:29 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:50:29.364Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[4f37afb][resolved]
Feb 02 02:50:59 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:50:59.364Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[1d2cb32][active] 
Feb 02 02:51:04 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:51:04.365Z caller=dispatch.go:473 component=dispatcher aggrGroup="{}/{severity=\"warning\"}/{environment=\"testing\"}:{alertname=\"DiskReplace\", environment=\"testing\", severity=\"warning\"}" msg=flushing alerts="[DiskReplace[4f37afb][resolved] DiskReplace[281d27a][resolved] DiskReplace[1d2cb32][active]]"
Feb 02 02:51:04 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:51:04.538Z caller=notify.go:685 component=dispatcher receiver=alerts-testing integration=webhook[0] msg="Notify success" attempts=1
Feb 02 02:51:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:51:44.366Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[281d27a][resolved]
Feb 02 02:51:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:51:44.366Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[4f37afb][resolved]Feb 02 02:52:14 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:52:14.366Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[1d2cb32][active] 
Feb 02 02:52:24 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:52:24.365Z caller=dispatch.go:473 component=dispatcher aggrGroup="{}/{severity=\"warning\"}/{environment=\"testing\"}:{alertname=\"DiskReplace\", environment=\"testing\", severity=\"warning\"}" msg=flushing alerts="[DiskReplace[4f37afb][resolved] DiskReplace[281d27a][resolved] DiskReplace[1d2cb32][active]]"
Feb 02 02:52:59 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:52:59.367Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[281d27a][resolved]Feb 02 02:52:59 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:52:59.367Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[4f37afb][resolved]
Feb 02 02:53:29 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:53:29.367Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[1d2cb32][active] 
Feb 02 02:53:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:53:44.365Z caller=dispatch.go:473 component=dispatcher aggrGroup="{}/{severity=\"warning\"}/{environment=\"testing\"}:{alertname=\"DiskReplace\", environment=\"testing\", severity=\"warning\"}" msg=flushing alerts="[DiskReplace[4f37afb][resolved] DiskReplace[281d27a][resolved] DiskReplace[1d2cb32][active]]"
Feb 02 02:54:14 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:54:14.369Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[281d27a][resolved]
Feb 02 02:54:14 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:54:14.369Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[4f37afb][resolved]
Feb 02 02:54:44 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:54:44.370Z caller=dispatch.go:138 component=dispatcher msg="Received alert" alert=DiskReplace[1d2cb32][active] Feb 02 02:55:04 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:55:04.365Z caller=dispatch.go:473 component=dispatcher aggrGroup="{}/{severity=\"warning\"}/{environment=\"testing\"}:{alertname=\"DiskReplace\", environment=\"testing\", severity=\"warning\"}" msg=flushing alerts="[DiskReplace[4f37afb][resolved] DiskReplace[281d27a][resolved] DiskReplace[1d2cb32][active]]"
Feb 02 02:55:04 pupa alertmanager[18119]: level=debug ts=2021-02-01T23:55:04.549Z caller=notify.go:685 component=dispatcher receiver=alerts-testing integration=webhook[0] msg="Notify success" attempts=1

Start by reproducing the grouped-alert behavior using the supplied repeat_interval configuration and logs, then trace aggregation-group handling around the dispatch.go entries and the change introduced by PR#2040. Check notify.go as needed; done means resolved alerts are removed after repeat_interval when a firing alert remains, with coverage for the reported sequence.