Alert on "users experiencing errors" not triggering, despite being over warning and error thresholds

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

I haven't been able to reproduce this error since it happened, but I have the broken alert still in my account if that would be helpful.

The alert settings are:
When: Users experiencing errors is above 10 in 5 minutes
Then: Send a Slack notification to

The query defined is
```
count_unique(user)
(event.type:[error, default]) AND (level:fatal release.stage:adopted) over 5 minutes
```

### Expected Result

I should be getting Slack messages whenever the warning or error thresholds are exceeded

### Actual Result

i'm not getting any messages. And the alert remains in a "resolved" state even though the timechart shows the counts being over threshold

### Product Area

Alerts

### Link

_No response_

### DSN

_No response_

### Version

_No response_