azure – Wrong Bill cause of SQL DB – free tier

I recently got charged $700 for a SQL DB plan that I had set as a free tier. I remember having checked the “free tier” option:

As soon as I got an SMS alert about the charge, I deleted everything. I also opened a ticket through the portal, and an operator is handling the refund for me.

Guide I followed

I’m worried about such situations and had previously set a budget alert:

Alerts

Alert detail

However, the alert linked to my subscription did not show up. I’d like to know why this happened and how to prevent similar issues in the future.

Thanks,

Luca