Hi,
changing the calendar of an event on an iOS-device is not properly handled by the server. I opened an issue 2 months ago on github (owncloud): https://github.com/owncloud/core/issues/23829
Since then nothing much happend, except it has been labeled as a bug.
The problem also exists in Nextcloud 9.0.50 (havent’t checked 9.0.51, since it seems only to contain “cosmetical” corrections). Though this is only a relatively small thing, it keeps me from recommending Nextcloud (or also Owncloud) to other people who are not as familiar with computers as I am.
I’d quite like to switch to nextcloud, because I like the idea behind it 
Should also open an issue at github/nextcloud as well? Don’t want to annoy the devs though with a doubling…
Cheers, Bernie_O