Calendar alert changing to 10 minutes before despite choosing no alert

I've found 1 potential fix, if you are using a Gmail based account.

• log in to your Gmail calendar via www.gmail.com
• When in Calendar view, go to settings
• On the left, scroll down to "Settings for my Calendar"
• Select "Event Notifications" and you will probably see a rule that says "Notification - 10 - Minutes"
• Delete it.

For some reason Gmail accounts have this rule included by default. I just found it in 3 of my Gmail accounts and was tearing my hair out when every new appointment I added on iPhone or Mac with NO alert kept automatically adding in a 10 minute alert. a few seconds after I created them.

Problem fixed now.