Remove occurrence broken.

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
Removing occurrences results in unusual behaviour, both through the event editor and the secondary action button. It appears to change the dtstart of the whole event. On reloading the view only the exception is shown (usually).

### To Reproduce
1. Add a repeating event
2. Remove an occurrence of the repeating event
3. Calendar duplicates events then deletes events leaving a different event.

### Expected behavior
The occurrence is removed and the rest of the occurrences are unaffected.

### Screenshots or screen recordings

Please check what applies:
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- [x] I'm using the latest version from git that I've manually compiled
- [ ] I'm using the latest released stable version

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