Calendar notifications disappearing on iOS 18.1

Since upgrading to iOS 18.1 I've noticed that iCloud Calendar event notifications have been disappearing after two or three days on my iPhone 15 Pro Max.

I've always had the notifications set up to appear a week prior to events, birthdays etc. This has always worked faultlessly. However, since moving to iOS 18.1 the banners have been disappearing from my Notification Centre.

I can set up a second notification to trigger two days before the event, which is a workaround of sorts, but this only works on my own calendars. I'm unable to set up a second notification for birthdays, for example.

I also use the Calendar app for my work calendar, which is a Microsoft 365 service. I'm not even able to add second notifications for events on this calendar. I set the notifications up for a week before events, but if I then add a second notification it deletes the instruction for the first notification.

None of these problems ever occurred prior to iOS 18.1. I've tried turning notifications off and on again, I've tried force-closing the app, rebooting my phone etc, but nothing has worked. Has anyone else encountered anything like this?