Did you check the defaults I mentioned?
If they are set, then an alert gets set whenever you create an event; automatically. Otherwise there are none. No alert, no notification.
While you can not set a 10 minute default, 10 minutes is the top choice when creating an alert. Chances are that you created your events with that alert; and something has changed so that new events do not have an alert (or the time has changed.
Also, I’m only talking about the default alerts for Apple’s Calendar app. Other apps may have different default settings. I typically use Fantastical, which can be set for a default of 10 minutes. Many thrid party calendar apps, like Fantastical, work with Apple’s calendar service; so both Calendar and the thrid party app have the same events.
I seem to remember 10 minutes being available as a default. Maybe Apple changed it, and a glitch caused settings to revert to no alert if the old time was not available. That’s pure speculation.
Anyway, the way forward is to create a couple of events with alerts (however you do it) and make sure new events work as expected. Then go back and review your important events to make sure they have alerts set.
