Hi,
i’ve the same problem on my side with the iphone 6 of my partner.
Every time he creates and invite someone else, the event is automatically canceled within the hour.
I’ve investigated many times in the logs or activity module and on his computer and phone. I didn’t found a clue.
I’ve found similar topic on GitHub about thi bug. https://github.com/nextcloud/calendar/issues/4748
Probably a bug with a caldav function.
Let us know if there is some progress about this.