Really? How? I spent 6 hours debugging this yesterday and it most certainly does not just work. Under some circumstances. Could you specify whether you had a ‘Personal’ calendar in your nc before setting up the phone? Are you using ‘Deck’ and or ‘Tasks’?
Testing another phone, account, now.
Using one of the calendars (the Personal that is default on nextcloud) from NC with a second account and other phone I have the same behaviour. None of my events from the phone’s default account get synced. But I’m debugging in another thread.