Which server do you use? Does it support recurring events with deleted instances? Please provide much more information: steps to reproduce, Web server and CalDAV/CardDAV server logs taken while the problem occurs, DAVdroid debug info and verbose DAVdroid logs as described in [READ BEFORE POSTING] What’s required to diagnose a problem.
Copy events between accounts?
If I create an appointment on my Android phone (LG V10, Android 7.0) using my Google account event, that new appointment will not show up in my nextcloud calendar. Appointments made on nextcloud have no trouble showing up on the phone. Isn’t DAVdroid supposed to be able sync between the two accounts without having to create duplicate entries via each event? Because I have a couple of scheduling services that don’t support CalDav, I have to use Google, and I need to know that all my calendars are synced.
No, DAVdroid synchronizes events which are stored in the DAVdroid account with your CalDAV server. It does not copy events between accounts or process events otherwise.
You need to choose the correct account in the drop down menu when creating a new event. Some manufacturer apps don’t have this ability but many other calendar apps can handle this.