"Clear All" works in an unexpected and non unintuitive way when it comes to calendar entries. Clicking it not only removes all notification entries from the panel but also all of the calendar entries which in my opinion is not what most users would expect as normally people wouldn't want their schedule for the day to just disappear if they just want to mark notifications as read.
I get it that technically the entries are just persistent notifications sent by the calendar application but I don't think this is the expected behavior from the users. Moreover, this is exactly what happens when if a user switches to any past or future dates in the calendar/notification panel. The calendar entries for that date are shown but it is not possible to remove them either individually or by clicking clear all (disabled). If it makes sense to not remove them for other days, why would we want to do it for the current day when the schedule is the most needed. To me it seems like the design is mixing up 2 different concepts here. A Notification panel and Today's schedule panel.
IMHO, a better design would have been to visually separate list of unread notifications and list of calendar events for the day like most operating systems do (macOS, Windows, Android, iOS). At the very least the clear all button should not touch the calendar entries.
If neither of these are acceptable or doable, perhaps a way to bring back calendar notification after they are cleared would work.
Thank you for taking the time to report this bug and helping to make Ubuntu better. The issue you are reporting is an upstream one and it would be nice if somebody having it could send the bug to the developers of the software by following the instructions at https:/