The “Include new, eligible accounts automatically” option allows Chase to send Plaid new accounts from their API, but it doesn’t add these accounts to the user’s Item.
When Plaid detects the presence of new accounts, a NEW_ACCOUNTS_AVAILABLE webhook will be fired to notify you that there are new accounts available to be linked.
To add these new accounts to their Item, the user will still need to permission them by going through the Account Select flow in Link's update mode. When initializing Link, set update.account_selection_enabled: true in the link_token configuration to surface the Account Select step.