Persistent red notification dot badge on iCal on iPhone

If your iPhone is not operating as expected, try a forced-restart - as this will often clear an isolated software issue:

Force restart iPhone - Apple Support

If the problem persists after performing the restart, it may be necessary to reset your iPhone settings. While this reset will not cause any loss of data, as a precaution, it is always wise to ensure that you have a current iCloud or iTunes backup before performing the reset:

How to back up your iPhone, iPad, and iPod touch - Apple Support

When you are happy to proceed, you can perform the reset from iPhone settings:

Settings > General > Transfer or Reset iPad > Reset > Reset All Settings

If the problem still doesn’t clear, the next step is to reset the iPhone to factory settings:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Otherwise, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and you can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple

Possibly a Siri suggestion for a calendar entry from a date in an email or similar. Click on the inbox tray down on the bottom right of the calendar window and see if it's there.

That seems to be where the issue is. It seems the service the dentist uses to send reminders is too good or something. Though why the one appointment of two scheduled at the time didn't give the same results is odd. The problem date Siri was stuck on has been deleted. and as of the last 10 minutes hasn't returned. I had thought I'd deleted it before but perhaps not. Still odd that there was a Siri suggestion for one and not the other.

Thanks.

Thanks, nothing there had worked. See below.