Microsoft365 calendar invites are being resent on backup

[tofublock]

I managed to track down a very unfortunate behaviour of my Microsoft365 calendar resending and cancelling meeting invites to attendees and was able to trace it back to the Nextcloud Android app's calendar auto-backup which uses this code. (See my issue there)
I tried backing up the same calendar with the Calendar Import Export app and same results: Attendees of calendar invites receive a cancellation, another invitation or first a cancellation, then a reinvitation. It seems like the backup is changing the entries somehow which triggers that behaviour.
Looking at SaveCalendar.java I haven't yet found anything that manipulates calendar entries, just reads them. Does anybody have an idea why this might be happening?

Thanks!