What do your Nextcloud / Web server logs say?
If there’s nothing useful, please ask in the Nextcloud forums, because this sounds like a server thing.
I’m using DAVx5 3.0 on Samsung S8 and Nextcloud 18.0.3. My contacts are out of sync. On NC I see only 100 contacts, but on my phone 802. I have tried sync from mobile several times, but not all contacts are synced to NC. Is there a way how to forces sync, or push all contacts from mobile to NC ? I can’t find such a function.
Please provide steps to reproduce, Web server and CalDAV/CardDAV server logs taken while the problem occurs, DAVx⁵ debug info (DAVx⁵ / Settings / Debug info) and verbose DAVx⁵ logs etc as described in [READ BEFORE POSTING] What’s required to diagnose a problem.