We were noticing some discrepancies in alert groups counts.
From a real example:
```
# queryset is the alert groups initial queryset
>>> qs = queryset.filter(channel_filter__alert_receive_channel=arc)
>>> qs.filter(resolved_at__isnull=False).count()
1318
>>> qs = queryset.filter(channel=arc)
>>> qs.filter(resolved_at__isnull=False).count()
1356
>>> qs.filter(resolved_at__isnull=False, channel_filter__isnull=True).count()
38
```
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