The problem is that syncing between devices is not implemented in KeePass itself but through an external tool (Nextcloud, Syncthing, or whatever else). The sync client will only see the ciphertext and won’t be able to tell which records have been changed, only that two different binary files have a common ancestor and are in conflict.

The most obvious solution is to lock and close the database when it’s not in use (which is a good practice from a security perspective too), and to sync immediately when it is changed.