No email is sent to the attendee
Firstly thanks for your work.
I am using DAVdroid from f-droid.org on my Fairphone Open with Nextcloud 10. It works fine but when I add an attendee no email is sent to the attendee.
3rd party issue, this should be handled by the server (Nextcloud).
I think you can tell us how you reproduce the issue and send verbose logs including the related ical content: https://forums.bitfire.at/topic/749/read-before-posting-what-s-required-to-diagnose-a-problem
The email address issue should have nothing to do with a sync adaptor in theory. The only possibility is that if your account name in DAVDroid is not an email address, the ORGANIZOR field in ical will be filled with invaild email address by Android, and may cause some unknown problems.
RSVP=TRUE, which is the same thing as CalDavSyncAdapater does.