Why don't all events show up when making a Calendar in Photos?

When making a calendar in Photos, I like to include national holidays as well as family birthdays. Those are imported in two separate iCal calendars. However, whenever there are events on the same day, only one of the calendar's events show up. So if someone was born on Memorial Day, it would show Memorial Day, not the birthday, but all the other un-conflicting events from the birthdays calendar would show up.

I assume this is a bug. Is there a way to work around it other than walking through every date in the birthdays calendar to make it show up?