When you group like this, and no documents match, there won't be any groups to alert on. Index threshold can't really be used for this sort of alerting. I suggest you look at metric threshold or custom threshold rule types instead, which offer some options for alerting on "no data" conditions.
You might want to change your condition to something that will match some documents. I fear it's again matching nothing, so there's nothing to group over. You can set the alerting action to only fire on No Data, and not have one set on Alert, so you could change your condition to IS ABOVE 1, for instance.
Interesting, I have tried your idea. I set action on No Data and have the condition modified to be ABOVE 150. It alerts my healthy node as expected but the no data action doesn't run still.
Here is my action setting.
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