YES - couple of issues with multiple accounts.
I have 6 accounts - all with email, 3 use Contacts-Main, Family, Work, 2 use Calendar-Main and Family.
After upgrade noticed bug first on Calendar. Have main account as default but the second Calendar is a shared so can put family events for us all to see - those calendar events started doubling with event on the shared calendar (NOT "YOUR FAMILY" calendar in Outlook but a separate account). This has worked perfect for years. Now show for each an event on that calendar and same event on my Main calendar. If try to delete the one on Main - both delete.
Then noticed Contacts and not all contacts that are on main account synching (and some being the contacts noted as IOS family).
Did - delete account on iphone and set up again and changed default settings back to Main. Worked for a bit.
Noticed again and settings had changed default.