group_by: [alertname, alertstate] doesn't work as it should

What did you do?
I used

group_by:
  - alertname
  - alertstate

config and got multiple alerts (both firing and resolved) mixed together in a single notification.

What did you expect to see?
Separate notifications: one for firing alerts and the other for resolved ones.

What did you see instead? Under which circumstances?
A single notification with multiple alerts (both firing and resolved) mixed together.

Environment
Yes.

• System information:

Linux 3.10.0-1127.el7.x86_64 x86_64

• 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 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:
  resolve_timeout: 5m
  smtp_from: x@y.com
  smtp_smarthost: 10.0.0.2:25
  smtp_require_tls: false

route:
  group_by:
    - alertname
    - alertstate
  group_wait: 30s
  group_interval: 1m
  repeat_interval: 3h
  receiver: topguys

receivers:
  - name: topguys
    email_configs:
      - to: z@y.com
        send_resolved: true

Start by reproducing the behavior with the supplied Alertmanager route configuration, especially group_by containing alertname and alertstate, with send_resolved enabled. Trace how the route groups firing and resolved alerts, then add coverage for the reported configuration; done means firing and resolved alerts arrive in separate notifications.