Calendar Alerts not syncing between iPad and iPhone or Mac

I am using the stock calendar app on all 3 devices, iPad, iPhone, and Mac. All updated to latest software.

I have a calendar I use as sort of a reminder list set up with default alerts. Sometimes as I complete a future item I will turn off the alert for the item.

For example on Wed Dec 18 I have an entry for "Pay cable bill" but I paid it today, I will open that entry and change the alert to "None". That works and syncs perfectly between the Mac and the iPhone. However, the iPad alert remains and on Wednesday I will get reminded to pay that bill.

Why does every other part of the calendar sync work correctly but the alerts seem to work independently on the iPad? I have to remember to go to the calendar on either the iPhone or Mac AND the iPad to turn off alerts I no longer need.