I have a custom-recurring event (for weekdays at 4pm) in which I have 1 alert for 5 minutes before. Usually, all my events have an alert set for 1 day before, but for this recurring event alone, I removed (or changed?) the default alert when I created it. I feel like it worked fine for years without issue, but for awhile now (say weeks/months), the default alert keeps getting re-added to the recurring event. I keep opening it up, removing it, and select 'apply to all future events'. I haven't worked out how frequently this happens, but it's at least happening once a week.
I tried ending the previous recurring event last week and created a new/fresh one, but the default alarm just popped up again! How the heck do I kill that alarm once-and-for-all without removing my default alarm (which I use for all other new events) and keep the reminder in the calendar app (because I occasionally edit a single event in the series).
I have submitted feedback to Apple about this issue.
Side-note: around the same time this started happening, an "Open File" alert for a separate weekday recurring event also kept getting messed up. The open file alert kept getting changed to the default alert at about the same incidence rate as the aforementioned event. I had isolated that issue to the fact that other synched devices didn't know what to do about a file they didn't have access to and mangled the event, so I figured out a way to move the event to a calendar that's located "On My Mac" and I wrote a blogpost about it. However, this is a basic alert, so that explanation doesn't cover this issue.