My problem may be similar. I have an event to remind me of property tax payments. The local gov allows a 4 payment plan starting end of Jan and every other month after that.
I have the calendar marked for the last weekday of the month for these payments and have the alarm set for 21 days in advance. But the alarm keeps triggering during the in between months. I've had this problem for years and figured I must be configuring it wrong.
In the pic I've hovered over the last day of the month to show the tax reminder is not there even though the alarm is triggering. The event shows correctly next month.