Multidimension Rule Alert (I see multiple alerts in preview, but only get one instance alert through web hook)

I’ve set a multi dimension rule (reduce, math):

And when I preview my alert:
I have multi alerts, (my query is summed by instance)

But through webhook, I only receive one alert, (one instance)

I am expecting multiple separate alerts like the preview shows me, any idea why I got only one?

p.s. I don’t have any matchers configured in the notification policy…