So I had a very nasty behaviour of my exchange work calendar which I synced into my Nextcloud calendar, and it would automatically cancel and recreate all events, which would resend invitations to attendees. It was very unprofessional, to say the least.

After months of this, I finally figured out that the culprit was the calendar auto-backup in the Android Nextcloud client. Turned it off, and my problems went away. Maybe you have something similar going on?