macOS Calendar cannot stop it generating alerts

I have an item in my Calendar on macOS 15.1 (iCloud synced) with a daily repeat.

It has an alert set for the default 15 mins from start. Works fine.

A few weeks ago I wanted to stop the alert without deleting the calendar item (so I could enable the alert again at a later date). So I opened the item, cleared the alert to "None", and choosing "All Future Events" from the popup so that it would not alert me any more.

This works fine .. for about a week .. and then suddenly the alerts start up again all by themselves, reset back to the default 15 mins before start. So I stop the alert AGAIN, and again it works for about a week .. then resets again.

If I check after stopping the alert all future events quite clearly show no alerts coming up .. even a month or two ahead. But sure enough after a week Calendar changes its mind and POW off they go again all by themselves.

Anyone have any idea what I'm doing wrong? Or is this a macOS bug ? TIA.